Add fiber type to better support custom fiber APIs

[kooldev]

Adds a "kind" field to zend_fiber_context to make sure that different fiber implementations do not get mixed up. The new field is set to zend_ce_fiber for PHP fibers and checked in Fiber::suspend() and Fiber::this() to ensure that these methods can only be called from within a running fiber.

The main reason for allowing different fiber implementations is to be prepared for async support that might come to PHP in a future version (or be provided by PHP extensions starting with PHP 8.1). Consider an example like this:

$task = async $client->sendRequest($request);
$response = await $task;

In this case the async expression should evaluate into a PHP object that is backed by a fiber but does not expose the same API as Fiber to userland. It would be possible to use the existing ZEND_API to create a Fiber and store that as a property of the new object, but that would be wasting memory for additional objects. While this is feasible in a userland implementation (due to lack of alternatives) it would not be reasonable to implement this in C. Having Fiber in PHP core is great and it should not be changed / restricted to async-only as there are other use cases for fiber (for example emitting values from deep within a callstack without turning everything into generators).

The great thing about this is that one can easily develop an async implementation as a PHP extension (of course without keyword support) and everybody has a chance to try and evaluate it before going for an RFC. It is also possible for Swoole to use the same approach and reuse some of the code that PHP 8.1 provides for their implementation.

[nikic]

Should this use zend_class_entry *, if a CE is used for identification?

[kooldev]

This is specified as void * to support fibers that are not exposed to PHP userland (pure C fibers) aswell hence restricting it to zend_class_entry * would be difficult due to no class entry being present. In essence this pointer just needs to be a unique address for each type of fiber. I decided to use zend_fiber_ce because it satisfies this requirement and is easy to associate with the fiber.