Implement Fiber GC handler

[kooldev]

Implements a GC handler to avoid memory leaks when a fiber callback (or result) holds a reference to the Fiber object. The following example produces a memory leak without proper GC handler:

$fiber = null;
$fiber = new Fiber(function () use (&$fiber) { });

The leak occurs because the Fiber is never started and it cannot be garbage collected due to the callback holding a reference to the Fiber object (this will also happen for transitive / nested references).

$fiber = null;
$fiber = new Fiber(function () use (&$fiber) {
   return $fiber;
});
$fiber->start();

This example will also leak memory due to the Fiber referencing itself from it's result. This PR addresses both leaks by providing a get_gc object handler and 2 additional tests that cover the demonstrated memory leaks.

[nikic]

Not a fan of the refactoring you did here. If the sole purpose was to get those zvals next to each other, then please use zend_get_gc_buffer instead.

[kooldev]

@nikic I simplified / fixed the PR as you suggested (I was not aware that zend_get_gc_buffer even existed). 😄