This change adds an `eval_breaker` field to `PyThreadState`, renaming the existing `eval_breaker` to
`interp_eval_breaker` (its uses are explained further down). The primary motivation is for
performance in free-threaded builds: with thread-local eval breakers, we can stop a specific thread
(e.g., for an async exception) without interrupting other threads.

There are still two situations where we want the first available thread to handle a request:
- Running a garbage collection: In normal builds, we set `_PY_GC_SCHEDULED_BIT` on the current
  thread. In case a thread suspends before handling the collection, the bit is copied to and from
  `interp_eval_breaker` on thread suspend and resume, respectively. In a free-threaded build, we
  simply iterate over all threads and set the bit. The first thread to check its eval breaker runs
  the collection, unsetting the bit on all threads.
  - Free-threaded builds could have multiple threads attempt a GC from one trigger if we get very
    unlucky with thread scheduling. I didn't put any protections against this in place because a)
    the consequences of it happening are just that one or more threads will check the GC thresholds
    right after a collection finishes, which won't affect correctness and b) it's incredibly,
    vanishingly unlikely.
- Pending calls not limited to the main thread (possible since python/cpython@757b402ea1c2). This is
  a little tricker, since the callback can be added from any thread, with or without the GIL
  held. If the targeted interpreter's GIL is locked, we signal the holding thread. When a thread is
  resumed, its `_PY_CALLS_TO_DO` bit is derived from the source of truth for pending calls (one of
  two `_pending_calls` structs). This handles situations where no thread held the GIL when the call
  was first added, or if the active thread did not handle the call before releasing the GIL. In a
  free-threaded build, all threads all signaled, similar to scheduling a GC.

The source of truth for the global instrumentation version is still in `interp_eval_breaker`, in
both normal and free-threaded builds. Threads usually read the version from their local
`eval_breaker`, where it continues to be colocated with the eval breaker bits, and the method for
keeping it up to date depends on build type. All builds first update the version in
`interp_eval_breaker`, and then:
- Normal builds update the version in the current thread's `eval_breaker`. When a thread takes the
  GIL, it copies the current version from `interp_eval_breaker` as part of the same operation that
  copies `_PY_GC_SCHEDULED_BIT`.
- Free-threaded builds again iterate over all threads in the current interpreter, updating the
  version on each one.
Instrumentation (and the specializing interpreter more generally) will need more work to be
compatible with free-threaded builds, so these changes are just intended to maintain the status quo
in normal builds for now.

Other notable changes are:
- The `_PY_*_BIT` macros now expand to the actual bit being set, rather than the bit's index. I
  think this is simpler overall. I also moved their definitions from `pycore_ceval.h` to
  `pycore_pystate.h`, since their main usage is on `PyThreadState`s now.
- Most manipulations of `eval_breaker` are done with a new pair of functions:
  `_PyThreadState_Signal()` and `_PyThreadState_Unsignal()`. Having two separate functions to
  set/unset a bit, rather than one function that takes the bit value to use, lets us use a single
  atomic `or`/`and`, rather than a loop around an atomic compare/exchange like the old
  `_Py_set_eval_breaker_bit` function.

Existing tests provide pretty good coverage for most of this functionality. The one new test I added
is to make sure a GC still happens if a thread schedules it then drops the GIL before the GC runs. I
don't love how complicated this test ended up so I'm open to other ideas for how to test this (or
other things to test in general).

…l build

…g convention

Minor conflicts from the new _PY_EVAL_EXPLICIT_MERGE_BIT flag.

This means that the instrumentation version was the only thing left in
`interp_eval_breaker`, so I renamed it and performed some related
simplification.