The main issue was cancellation of fiber using `cancel` rather than
`ev/cancel` could cause issues with the event loop internal ref count.
Since this is almost certainly bad usage (and is not something I want to
encourage or support), we will warn against trying to resume or error
fibers that have already been suspended or scheduled on the event loop.
The distinction between "task" fibers and normal fibers is now kept by a
flag that is set when a fiber is resumed - if it is the outermost fiber
on the stack, it is considered a root fiber. All fibers scheduled with
ev/go or by the event loop are root fibers, and thus cannot be cancelled
or resumed with `cancel` or `resume` - instead, use `ev/cancel` or
`ev/go`.
Rather than manual reference counting for suspended fibers, we
automate the process by incrementing "extra_listeners" every time
we suspend a fiber in the event loop, and decrement when that fiber
is resumed. In this manner, we keep track of the number of suspending
fibers in a simpler, more correct way.
Lazy verification makes it easier to not leave funcenvs
in an invalid state, as well as be more precise with the validation.
We needed to verify the FuncEnvs actually pointed to a stack frame if
they were of the "on-stack" variant. There was some minor checking
before, but it was not enough to prevent func envs from pointing to
memory that was off of the fiber stack, overlapping stack frames, etc.
If possible, this will reduce the need to marshal fibers
in many cases. Also add this logic to the GC so holding a closure
that originally came from a fiber that crashed does not cause that fiber
to hang around forever.
While C functions are not re-entrant, signaling from a C function
can be used to implement async returns. When resuming a fiber that
signalled from within a C function, the fiber is started after the
instruction that emitted the signal. The resume argument is used
as the return result from the c function.
