This patch introduces thread specific storage APIs
for tools which use `rb_internal_thread_event_hook` APIs.

* `rb_internal_thread_specific_key_create()` to create a tool specific
  thread local storage key and allocate the storage if not available.
* `rb_internal_thread_specific_set()` sets a data to thread and tool
  specific storage.
* `rb_internal_thread_specific_get()` gets a data in thread and tool
  specific storage.

Note that `rb_internal_thread_specific_get|set(thread_val, key)`
can be called without GVL and safe for async signal and safe for
multi-threading (native threads). So you can call it in any internal
thread event hooks. Further more you can call it from other native
threads. Of course `thread_val` should be living while accessing the
data from this function.

Note that you should not forget to clean up the set data.

This entirely changes how it is tested. Rather than to use counters
we now record the timeline of events with associated threads which
makes it much easier to assert that certains events are only preceded
by a specific event, and makes it much easier to debug unexpected
timelines.

Co-Authored-By: Étienne Barrié <etienne.barrie@gmail.com>
Co-Authored-By: JP Camara <jp@jpcamara.com>
Co-Authored-By: John Hawthorn <john@hawthorn.email>

Context: https://github.com/ivoanjo/gvl-tracing/pull/4

Some hooks may want to collect data on a per thread basis.
Right now the only way to identify the concerned thread is to
use `rb_nativethread_self()` or similar, but even then because
of the thread cache or MaNy, two distinct Ruby threads may report
the same native thread id.

By passing `thread->self`, hooks can use it as a key to store
the metadata.

NB: Most hooks are executed outside the GVL, so such data collection
need to use a thread-safe data-structure, and shouldn't use the
reference in other ways from inside the hook.

They must also either pin that value or handle compaction.

Except for the `RESUMED` event.

[Misc #18891]

[Feature #18339]

After experimenting with the initial version of the API I figured there is a need
for an exit event to cleanup instrumentation data. e.g. if you record data in a
{thread_id -> data} table, you need to free associated data when a thread goes away.

Ref: https://bugs.ruby-lang.org/issues/18339

Design:

- This tries to minimize the overhead when no hook is registered.
  It should only incur an extra unsynchronized boolean check.
- The hook list is protected with a read-write lock as to cause
  contention when some hooks are registered.
- The hooks MUST be thread safe, and MUST NOT call into Ruby as they
  are executed outside the GVL.
- It's simply a noop on Windows.

API:

```
rb_internal_thread_event_hook_t * rb_internal_thread_add_event_hook(rb_internal_thread_event_callback callback, rb_event_flag_t internal_event, void *user_data);
bool rb_internal_thread_remove_event_hook(rb_internal_thread_event_hook_t * hook);
```

You can subscribe to 3 events:

  - READY: called right before attempting to acquire the GVL
  - RESUMED: called right after successfully acquiring the GVL
  - SUSPENDED: called right after releasing the GVL.

The hooks MUST be threadsafe, as they are executed outside of the GVL, they also MUST NOT call any Ruby API.

Must not be a bad idea to improve documents. [ci skip]