Add //pkg/syncevent.

Package syncevent is intended to subsume ~all uses of channels in the sentry
(including //pkg/waiter), as well as //pkg/sleep.

Compared to channels:

- Delivery of events to a syncevent.Receiver allows *synchronous* execution of
  an arbitrary callback, whereas delivery of events to a channel requires a
  goroutine to receive from that channel, resulting in substantial scheduling
  overhead. (This is also part of the motivation for the waiter package.)

- syncevent.Waiter can wait on multiple event sources without the high O(N)
  overhead of select. (This is the same motivation as for the sleep package.)

Compared to the waiter package:

- syncevent.Waiters are intended to be persistent (i.e. per-kernel.Task), and
  syncevent.Broadcaster (analogous to waiter.Queue) is a hash table rather than
  a linked list, such that blocking is (usually) allocation-free.

- syncevent.Source (analogous to waiter.Waitable) does not include an equivalent
  to waiter.Waitable.Readiness(), since this is inappropriate for transient
  events (see e.g. //pkg/sentry/kernel/time.ClockEventSource).

Compared to the sleep package:

- syncevent events are represented by bits in a bitmask rather than discrete
  sleep.Waker objects, reducing overhead and making it feasible to broadcast
  events to multiple syncevent.Receivers.

- syncevent.Receiver invokes an arbitrary callback, which is required by the
  sentry's epoll implementation. (syncevent.Waiter, which is analogous to
  sleep.Sleeper, pairs a syncevent.Receiver with a callback that wakes a
  waiting goroutine; the implementation of this aspect is nearly identical to
  that of sleep.Sleeper, except that it represents *runtime.g as unsafe.Pointer
  rather than uintptr.)

- syncevent.Waiter.Wait (analogous to sleep.Sleeper.Fetch(block=true)) does not
  automatically un-assert returned events. This is useful in cases where the
  path for handling an event is not the same as the path that observes it, such
  as for application signals (a la Linux's TIF_SIGPENDING).

- Unlike sleep.Sleeper, which Fetches Wakers in the order that they were
  Asserted, the event bitmasks used by syncevent.Receiver have no way of
  preserving event arrival order. (This is similar to select, which goes out of
  its way to randomize event ordering.)

The disadvantage of the syncevent package is that, since events are represented
by bits in a uint64 bitmask, each syncevent.Receiver can "only" multiplex
between 64 distinct events; this does not affect any known use case.

Benchmarks:

BenchmarkBroadcasterSubscribeUnsubscribe
BenchmarkBroadcasterSubscribeUnsubscribe-12         	45133884	        26.3 ns/op
BenchmarkMapSubscribeUnsubscribe
BenchmarkMapSubscribeUnsubscribe-12                 	28504662	        41.8 ns/op
BenchmarkQueueSubscribeUnsubscribe
BenchmarkQueueSubscribeUnsubscribe-12               	22747668	        45.6 ns/op
BenchmarkBroadcasterSubscribeUnsubscribeBatch
BenchmarkBroadcasterSubscribeUnsubscribeBatch-12    	31609177	        37.8 ns/op
BenchmarkMapSubscribeUnsubscribeBatch
BenchmarkMapSubscribeUnsubscribeBatch-12            	17563906	        62.1 ns/op
BenchmarkQueueSubscribeUnsubscribeBatch
BenchmarkQueueSubscribeUnsubscribeBatch-12          	26248838	        46.6 ns/op
BenchmarkBroadcasterBroadcastRedundant
BenchmarkBroadcasterBroadcastRedundant/0
BenchmarkBroadcasterBroadcastRedundant/0-12         	100907563	        11.8 ns/op
BenchmarkBroadcasterBroadcastRedundant/1
BenchmarkBroadcasterBroadcastRedundant/1-12         	85103068	        13.3 ns/op
BenchmarkBroadcasterBroadcastRedundant/4
BenchmarkBroadcasterBroadcastRedundant/4-12         	52716502	        22.3 ns/op
BenchmarkBroadcasterBroadcastRedundant/16
BenchmarkBroadcasterBroadcastRedundant/16-12        	20278165	        58.7 ns/op
BenchmarkBroadcasterBroadcastRedundant/64
BenchmarkBroadcasterBroadcastRedundant/64-12        	 5905428	       205 ns/op
BenchmarkMapBroadcastRedundant
BenchmarkMapBroadcastRedundant/0
BenchmarkMapBroadcastRedundant/0-12                 	87532734	        13.5 ns/op
BenchmarkMapBroadcastRedundant/1
BenchmarkMapBroadcastRedundant/1-12                 	28488411	        36.3 ns/op
BenchmarkMapBroadcastRedundant/4
BenchmarkMapBroadcastRedundant/4-12                 	19628920	        60.9 ns/op
BenchmarkMapBroadcastRedundant/16
BenchmarkMapBroadcastRedundant/16-12                	 6026980	       192 ns/op
BenchmarkMapBroadcastRedundant/64
BenchmarkMapBroadcastRedundant/64-12                	 1640858	       754 ns/op
BenchmarkQueueBroadcastRedundant
BenchmarkQueueBroadcastRedundant/0
BenchmarkQueueBroadcastRedundant/0-12               	96904807	        12.0 ns/op
BenchmarkQueueBroadcastRedundant/1
BenchmarkQueueBroadcastRedundant/1-12               	73521873	        16.3 ns/op
BenchmarkQueueBroadcastRedundant/4
BenchmarkQueueBroadcastRedundant/4-12               	39209468	        31.2 ns/op
BenchmarkQueueBroadcastRedundant/16
BenchmarkQueueBroadcastRedundant/16-12              	10810058	       105 ns/op
BenchmarkQueueBroadcastRedundant/64
BenchmarkQueueBroadcastRedundant/64-12              	 2998046	       376 ns/op
BenchmarkBroadcasterBroadcastAck
BenchmarkBroadcasterBroadcastAck/1
BenchmarkBroadcasterBroadcastAck/1-12               	44472397	        26.4 ns/op
BenchmarkBroadcasterBroadcastAck/4
BenchmarkBroadcasterBroadcastAck/4-12               	17653509	        69.7 ns/op
BenchmarkBroadcasterBroadcastAck/16
BenchmarkBroadcasterBroadcastAck/16-12              	 4082617	       260 ns/op
BenchmarkBroadcasterBroadcastAck/64
BenchmarkBroadcasterBroadcastAck/64-12              	 1220534	      1027 ns/op
BenchmarkMapBroadcastAck
BenchmarkMapBroadcastAck/1
BenchmarkMapBroadcastAck/1-12                       	26760705	        44.2 ns/op
BenchmarkMapBroadcastAck/4
BenchmarkMapBroadcastAck/4-12                       	11495636	       100 ns/op
BenchmarkMapBroadcastAck/16
BenchmarkMapBroadcastAck/16-12                      	 2937590	       343 ns/op
BenchmarkMapBroadcastAck/64
BenchmarkMapBroadcastAck/64-12                      	  861037	      1344 ns/op
BenchmarkQueueBroadcastAck
BenchmarkQueueBroadcastAck/1
BenchmarkQueueBroadcastAck/1-12                     	19832679	        55.0 ns/op
BenchmarkQueueBroadcastAck/4
BenchmarkQueueBroadcastAck/4-12                     	 5618214	       189 ns/op
BenchmarkQueueBroadcastAck/16
BenchmarkQueueBroadcastAck/16-12                    	 1569980	       713 ns/op
BenchmarkQueueBroadcastAck/64
BenchmarkQueueBroadcastAck/64-12                    	  437672	      2814 ns/op
BenchmarkWaiterNotifyRedundant
BenchmarkWaiterNotifyRedundant-12                   	650823090	         1.96 ns/op
BenchmarkSleeperNotifyRedundant
BenchmarkSleeperNotifyRedundant-12                  	619871544	         1.61 ns/op
BenchmarkChannelNotifyRedundant
BenchmarkChannelNotifyRedundant-12                  	298903778	         3.67 ns/op
BenchmarkWaiterNotifyWaitAck
BenchmarkWaiterNotifyWaitAck-12                     	68358360	        17.8 ns/op
BenchmarkSleeperNotifyWaitAck
BenchmarkSleeperNotifyWaitAck-12                    	25044883	        41.2 ns/op
BenchmarkChannelNotifyWaitAck
BenchmarkChannelNotifyWaitAck-12                    	29572404	        40.2 ns/op
BenchmarkSleeperMultiNotifyWaitAck
BenchmarkSleeperMultiNotifyWaitAck-12               	16122969	        73.8 ns/op
BenchmarkWaiterTempNotifyWaitAck
BenchmarkWaiterTempNotifyWaitAck-12                 	46111489	        25.8 ns/op
BenchmarkSleeperTempNotifyWaitAck
BenchmarkSleeperTempNotifyWaitAck-12                	15541882	        73.6 ns/op
BenchmarkWaiterNotifyWaitMultiAck
BenchmarkWaiterNotifyWaitMultiAck-12                	65878500	        18.2 ns/op
BenchmarkSleeperNotifyWaitMultiAck
BenchmarkSleeperNotifyWaitMultiAck-12               	28798623	        41.5 ns/op
BenchmarkChannelNotifyWaitMultiAck
BenchmarkChannelNotifyWaitMultiAck-12               	11308468	       101 ns/op
BenchmarkWaiterNotifyAsyncWaitAck
BenchmarkWaiterNotifyAsyncWaitAck-12                	 2475387	       492 ns/op
BenchmarkSleeperNotifyAsyncWaitAck
BenchmarkSleeperNotifyAsyncWaitAck-12               	 2184507	       518 ns/op
BenchmarkChannelNotifyAsyncWaitAck
BenchmarkChannelNotifyAsyncWaitAck-12               	 2120365	       562 ns/op
BenchmarkWaiterNotifyAsyncWaitMultiAck
BenchmarkWaiterNotifyAsyncWaitMultiAck-12           	 2351247	       494 ns/op
BenchmarkSleeperNotifyAsyncWaitMultiAck
BenchmarkSleeperNotifyAsyncWaitMultiAck-12          	 2205799	       522 ns/op
BenchmarkChannelNotifyAsyncWaitMultiAck
BenchmarkChannelNotifyAsyncWaitMultiAck-12          	 1238079	       928 ns/op

Updates #1074

PiperOrigin-RevId: 295834087

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