Previously, vm_make_env_each() (used during proc
creation and for the debug inspector C API) picked up the
non-GC-allocated iseq that rb_vm_push_frame_fname() creates,
which led to a SEGV when the GC tried to mark the non GC object.

Put a real iseq imemo instead. Speed should be about the same since
the old code also did a imemo allocation and a malloc allocation.

Real iseq allows ironing out the special-casing of dummy frames in
rb_execution_context_mark() and rb_execution_context_update(). A check
is added to RubyVM::ISeq#eval, though, to stop attempts to run dummy
iseqs.

[Bug #21180]

Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>

[Bug #20907]

Redirect `rb_nogvl` blocking operations to the fiber scheduler if possible
to prevent stalling the event loop.

[Feature #20876]

This reverts some of commit 87fb44dff6409a19d12052cf0fc07ba80a4c45ac.

We will rename and propose a slightly different interface.

If one thread is reading and another closes that socket, the close
blocks waiting for the read to abort cleanly. This ensures that Ruby is
totally done with the file descriptor _BEFORE_ we tell the OS to close
and potentially re-use it.

When the read is correctly terminated, the close should be unblocked.
That currently works if closing is happening on a thread, but if it's
happening on a fiber with a fiber scheduler, it does NOT work.

This patch ensures that if the close happened in a fiber scheduled
thread, that the scheduler is notified that the fiber is unblocked.

[Bug #20723]

This reverts commit dfa0897de89251a631a67460b941cd24a14c9b55.

This commit accidentally included some change in `parse.h`. Reverting
and re-applying the relevant changes.