maitake: consider merging `wait` and `sync` modules

[hawkw]

currently, maitake has a wait module, which contains WaitCell, WaitQueue, WaitMap, and (as of #301) Semaphore. in addition, it has a separate sync module, which currently contains Mutex and nothing else (and may grow a RwLock eventually/soon). this feels a bit unfortunate — the wait types are also synchronization primitives, so perhaps these modules should be unified into one sync module.