gh-108724: Add PyMutex and _PyParkingLot APIs

[colesbury]

PyMutex is a one byte lock with fast, inlineable lock and unlock functions for the common uncontended case. The design is based on WebKit's WTF::Lock.

PyMutex is built using the _PyParkingLot APIs, which provides a cross-platform futex-like API (based on WebKit's WTF::ParkingLot). This internal API will be used for building other synchronization primitives used to implement PEP 703, such as one-time initialization and events.

This also includes tests and a mini benchmark in Tools/lockbench/lockbench.py to compare with the existing PyThread_type_lock.

Uncontended acquisition + release:
Linux (x86-64): PyMutex: 11 ns, PyThread_type_lock: 44 ns
macOS (arm64): PyMutex: 13 ns, PyThread_type_lock: 18 ns
Windows (x86-64): PyMutex: 13 ns, PyThread_type_lock: 38 ns

PR Overview

The primary purpose of this PR is to implement PyMutex, but there are a number of support pieces (described below).

• PyMutex: A 1-byte lock that doesn't require memory allocation to initialize and is generally faster than the existing PyThread_type_lock. The API is internal only for now.
• _PyParking_Lot: A futex-like API based on the API of the same name in WebKit. Used to implement PyMutex.
• _PyRawMutex: A word sized lock used to implement _PyParking_Lot
• PyEvent: a one time event. This was used a bunch in the "nogil" fork and is useful for testing the PyMutex implementation, so I've included it as part of the PR.
• pycore_llist.h: Defines common operations on doubly-linked list. Not strictly necessary (could do the list operations manually), but they come up frequently in the "nogil" fork. (Similar to https://man.freebsd.org/cgi/man.cgi?queue)

Linked issue

• Issue: Add lightweight locking C API #108724