Update `ReentrantLock` implementation, add `CURRENT_ID` thread local.

This changes `ReentrantLock` to use `ThreadId` for the thread ownership check instead of the address of a thread local. Unlike TLS blocks, `ThreadId` is guaranteed to be unique across the lifetime of the process, so if any thread ever terminates while holding a `ReentrantLockGuard`, no other thread may ever acquire that lock again.

On platforms with 64-bit atomics, this is a very simple change. On other platforms, the approach used is slightly more involved, as explained in the module comment.

This also adds a `CURRENT_ID` thread local in addition to the already existing `CURRENT`. This allows us to access the current `ThreadId` without the relatively heavy machinery used by `thread::current().id()`.

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