* Avoiding using `Tempfile` which was retaining the file preventing it from unlinking.

* Restore experimental warnings.

* Documentation and code structure improvements.

* Improved validation of flags, clarified documentation of argument handling.

* Remove inconsistent use of `Example:` and add example to `null?`.

* Expose `IO::Buffer#private?` and add test.

Co-authored-by: Dustin Brown <dbrown9@gmail.com>

This is a C API for extensions to resolve and get function symbols of other extensions.
Extensions can check the expected symbol is correctly loaded and accessible, and
use it if it is available.
Otherwise, extensions can raise their own error to guide users to setup their
environments correctly and what's missing.

for future extensions.

Our current implementation of rb_postponed_job_register suffers from
some safety issues that can lead to interpreter crashes (see bug #1991).
Essentially, the issue is that jobs can be called with the wrong
arguments.

We made two attempts to fix this whilst keeping the promised semantics,
but:
  * The first one involved masking/unmasking when flushing jobs, which
    was believed to be too expensive
  * The second one involved a lock-free, multi-producer, single-consumer
    ringbuffer, which was too complex

The critical insight behind this third solution is that essentially the
only user of these APIs are a) internal, or b) profiling gems.

For a), none of the usages actually require variable data; they will
work just fine with the preregistration interface.

For b), generally profiling gems only call a single callback with a
single piece of data (which is actually usually just zero) for the life
of the program. The ringbuffer is complex because it needs to support
multi-word inserts of job & data (which can't be atomic); but nobody
actually even needs that functionality, really.

So, this comit:
  * Introduces a pre-registration API for jobs, with a GVL-requiring
    rb_postponed_job_prereigster, which returns a handle which can be
    used with an async-signal-safe rb_postponed_job_trigger.
  * Deprecates rb_postponed_job_register (and re-implements it on top of
    the preregister function for compatability)
  * Moves all the internal usages of postponed job register
    pre-registration

This can already be emulated by doing an atomic fetch_add of zero, but
this is more explicit.

[Bug #19994]