Add suspension listeners

[trowski]

This PR allows user-definable hooks to be invoked when a fiber (or main) is suspended or resumed.

The motivation is to allow libraries/apps to implement behavior such as that requested in amphp/amp#354 if so desired.

This is designed to be lightweight so applications that do not use these hooks do not incur a large performance overhead.

• Benchmarks to see what effect this has on performance.

[kelunik]

This breaks with the additional suspensions we've introduced for event loop callbacks now. I think this should be solved with a core / extension API in PHP itself rather than Revolt.

[trowski]

The event loop callback fiber does not break this mechanism. The fiber switch when invoking an event loop callback is essentially considered part of the event loop fiber from the perspective of this API. Entering the callback is similar to entering a new fiber created by launch(). It is the responsibility of the user to create a new context at that point if they intend to suspend.

However, addressing it from within PHP itself may be a better solution.

As this does not introduce a BC break, we are free to introduce this much later as we explore our options.

[trowski]

Note that I'm not entirely sure this is necessary myself. I wanted to propose a solution though and see what others thought.

[kelunik]

#23 converts Suspension into an interface, so listeners can now be implemented as decorators without shipping built-in support.

[trowski]

Closing this as it can be implemented in a separate library.