Flexible fiber bailout handling

[kooldev]

This PR moves struct and functions needed to capture / restore Zend VM state into zend_fibers.c to reduce API exposure. It was never possible to gain access to that state outside of zend_fiber_switch_context() (state is stored on the C stack), hence removing this from the header makes sense.

The reason for not calling zend_bailout() immediately in zend_fiber_switch_context() is that fiber implementations may require additional cleanup actions to be performed before bailing out. If we bailout during switch context it requires every call to zend_fiber_switch_context() to be wrapped in zend_try / zend_catch to dealt with that which is not ideal. Plus there is only a single call to that function in the implementation of Fiber that does direct forwarding of bailouts.

@trowski As always: Ping 😉

[trowski]

Should flags continue to be part of zend_fiber_context? With the bailout flag removed from zend_fiber_switch_context(), none of the context functions refer to flags anymore. Should it be moved to zend_fiber instead and let other implementations handle flags however they want/need?

[kooldev]

I moved flags from zend_fiber_context to zend_fiber and introduced a new fiber transfer flag ZEND_FIBER_TRANSFER_FLAG_BAILOUT. This is needed in order to communicate a bailout to other fibers, they will need to do some cleanup and bailout as well. It makes sense to use the transfer struct for that because it can be used across all fiber implementations.

[krakjoe]

Can you put the vm state changes in a separate PR please, they look unrelated.