The `call()` function allows invoking functions with a modified `this` context
and global environment. It's main use case is binding global variables for
dynamiclly loaded code at runtime.

Returns `null` if the given function value is not callable.
Returns the value returned by the invoked function in all other cases.


Test modifying `this` context

-- Testcase --
{%
	let o1 = {
		name: "Object #1",
		func: function() {
			print(`This is ${this.name}\n`);
		}
	};

	let o2 = {
		name: "Object #2"
	};

	o1.func();
	call(o1.func, o2);
%}
-- End --

-- Expect stdout --
This is Object #1
This is Object #2
-- End --


Test modifying environment

-- Testcase --
{%
	function fn() {
		print("Hello world\n");
	}

	fn();
	call(fn, null, { print: (s) => printf("Overridden print(): %s", s) });
%}
-- End --

-- Expect stdout --
Hello world
Overridden print(): Hello world
-- End --


Test isolating environment

-- Testcase --
{%
	function fn() {
		print("Hello world\n");
	}

	fn();
	call(fn, null, proto({}, {})); // should fail due to unavailable print
%}
-- End --

-- Expect stdout --
Hello world
-- End --

-- Expect stderr --
Type error: left-hand side is not a function
In fn(), line 3, byte 24:
  called from function call ([C])
  called from anonymous function ([stdin]:7:30)

 `        print("Hello world\n");`
  Near here -------------------^


-- End --


Test passing through arguments

-- Testcase --
{%
	function fn(a, b) {
		printf("The product of %d * %d is %d\n", a, b, a * b);
	}

	fn(3, 4);
	call(fn, null, null, 5, 6);
	call((...args) => printf("Args: %J\n", args), null, null, 1, 2, 3, 4, 5, 6);
%}
-- End --

-- Expect stdout --
The product of 3 * 4 is 12
The product of 5 * 6 is 30
Args: [ 1, 2, 3, 4, 5, 6 ]
-- End --