The generic implementation is based on the lock pool. A list of condition
variables (or waiting futexes) is added per lock. Basically, the lock
pool serves as a global hash table, where each lock represents
a bucket and each wait state is an element. Every wait operation
allocates a wait state keyed on the pointer to the atomic object. Notify
operations look up the wait state by the atomic pointer and notify
the condition variable/futex. The corresponding lock needs to be acquired
to protect the wait state list during all wait/notify operations.
Backends not involving the lock pool are going to be added later.
The implementation of wait operation extends the C++20 definition in that
it returns the newly loaded value instead of void. This allows the caller
to avoid loading the value himself.
The waiting/notifying operations are not address-free. Address-free variants
will be added later.
Added tests for the new operations and refactored existing tests for atomic
operations. Added docs for the new operations.
