This fixes a regression from changes to janet_try. In some cases, we
would not update the status of a fiber when signaling, which left the
fiber's status as whatever it had previously. This could lead to strange
control flow issues.
Rather than trying to be clever with pinning/unpinning, always
mark the root fiber and that should serve as thei singular common root in almost
all cases.
Do not restore pc when returning from top most fiber frame.
Also add JANET_DEBUG config define for various debugging related
configurations. In fiber.c, when debug is enabled we reallocate the
entire stack everytime we push a frame to help uncover use after free
errors.
Before, these bindings we just ignored. However, it useful for
controlling janet_printf and janet_eprintf, for example. These can
be called from C code without being inside a call to janet_continue.
This makes these operatios use constant stack space rather
than linear stackspace given the size of the inputs. This is important
to prevent certain parser input from causing a stack overflow - in
general, we try to avoid unbounded recursion.
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.
These additions, along with the change that user signals 0-4 cannot
be resumed, allow delimited continuation semantics, while repsecting
existing forms like `defer`, `with`, `with-vars`, etc.
Required a few changes to APIs, namely janet_root_fiber()
to get topmost fiber that is active in the current scheduler.
This is distinct from janet_current_fiber(), which gets the bottom
most fiber in the fiber stack - it might have a parent, and so cannot
be reliably resumed.
This is the kind of situation that makes symmetric coroutines more
attractive.
