concurrent iteration. (This is prep work for enabling specialization of
FOR_ITER in free-threaded builds.) The basic premise is:

 - Iterating over a shared _iterable_ (list, tuple or range) should be safe,
   not involve data races, and behave like iteration normally does.

 - Using a shared _iterator_ should not crash or involve data races, and
   should only produce items regular iteration would produce. It is _not_
   guaranteed to produce all items, or produce each item only once.

Providing stronger guarantees is possible for some of these iterators, but
it's not always straight-forward and can significantly hamper the common
case. Since iterators in general aren't shared between threads, and it's
simply impossible to concurrently use many iterators (like generators),
better to make sharing iterators without explicit synchronization clearly
wrong.

Specific issues fixed in order to make the tests pass:

 - List iteration could occasionally crash when a shared list wasn't already
   marked as shared when reallocated.

 - Tuple iteration could occasionally crash when the iterator's reference to
   the tuple was cleared on exhaustion. Like with list iteration, in
   free-threaded builds we can't safely and efficiently clear the iterator's
   reference to the iterable (doing it safely would mean extra, slow
   refcount operations), so just keep the iterable reference around.

 - Fast range iterators (for integers that fit in C longs) shared between
   threads would sometimes produce values past the end of the range, because
   the iterators use two bits of state that we can't efficiently update
   atomically. Rewriting the iterators to have a single bit of state is
   possible, but probably means more math for each iteration and may not be
   worth it.

 - Long range iterators (for other numbers) shared between threads would
   crash catastrophically in a variety of ways. This now uses a critical
   section. Rewriting this to be more efficient is probably possible, but
   since it deals with arbitrary Python objects it's difficult to get right.

There seem to be no more exising races in list_get_item_ref, so drop it from
the tsan suppression list.

actually correct and the real problem was an incorrect assert. The fast path
still contains notionally unsafe uses of memcpy/memmove, so add
list_get_item_ref back to the TSan suppressions file.

iterators, and fix build failures because labels can't technically be at the
end of compound statements (a statement must follow the label, even if it's
empty).

…lloc

…len and

tupleiter_setstate appropriately relaxed.

iterator test to accept the current behaviour of the fast range iterator and
avoid testing the (very unsafe) long range iterator, use threading.Barrier
where appropriate, and add comments to the unsafe uses of memcpy/memmove in
free-threaded builds.

relaxed atomics. (These are not performance-sensitive parts of the code, and
this lets us detect races elsewhere involving the iterators.)

…lloc