Run schedulers on uncaught exception

Fix closure scoping

Error if the wrong scheduler resumes a fiber

Fix fiber status on error

Fix garbage collection of fiber

Cache closure

Move scheduler creation to function

Update for typedef name change in php-src

Maintained compatibility with prior versions with a define.

Redesign Future as Awaitable

Delete ref to fiber when fiber finishes

Rename Scheduler to FiberScheduler

Use new observer API for finishing schedulers

Forbid resume outside scheduler

This keeps the suspend/switch stack consistent.

Reverse iterate through schedulers

Execute schedulers to completion

Fix switching resuming fiber from root context

Do not use pointer for fiber value

Update stub

Remove double-free of root context

Limit fiber creation to Scheduler::run()

Make Scheduler an interface

Scheduler fiber is created by calling Scheduler::run().

Return callback result from Fiber::run()

Add -Werror

Fix bogus assert

WIP: Scheduler::pause()