refac(maitake): combine wait module into sync

[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.

this branch moves all of maitake::wait into maitake::sync. in
addition,i fixed up the docs a little bit.

closes #302