Instead of setting a flag, each interrupt increments an atomic
counter. When the interrupt is finally handled, either by scheduling
code to run on the event loop or executing some out of band code, the
user must now decrement the interrupt counter with
janet_interpreter_interrupt_handled. While this counter is non-zero, the
event loop will not enter the interpreter. This changes the API a bit but
makes it possible and easy to handle signals without race conditions
or scheduler hacks, as the runtime can ensure that high priority code is
run before re-entering possibly blocking interpreter code again.
Also included is a new function janet_schedule_soon, which prepends to
the task queue instead of appending, allowing interrupt handler to skip
ahead of all other scheduled fibers.
Lastly, also update meson default options to include the
interpreter_interrupt code and raise a runtime error if os/sigaction
is used with interpreter interrupt but that build option is not enabled.
This would allow an embedder to suspend the current Janet fiber
via an external event like a signal, other thread, or really anything.
This is a useful primitive for custom schedulers that would call
janet_interpreter_interupt periodically (say, in an interval with SIG_ALRM),
do some work, and then use janet_continue on the janet_root_fiber, or
for embedding into other soft-realtime applications like a game. To say,
only allow about 5ms per frame of interpreter time.
In some cases, one might want to disable what is currently
SipHash for speed / better security mechansims. For example, using
red black trees for caches rather than hash tables.
