Simplify the pipe implementation.

- Remove the pipe package's dependence on the buffer package, which becomes
  unused as a result. The buffer package is currently intended to serve two use
  cases, pipes and temporary buffers, and does neither optimally as a result;
  this change facilitates retooling the buffer package to better serve the
  latter.

- Pass callbacks taking safemem.BlockSeq to the internal pipe I/O methods,
  which makes most callbacks trivial.

- Fix VFS1's splice() and tee() to immediately return if a pipe returns a
  partial write.

PiperOrigin-RevId: 351911375

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