atomic changes
extra: Make room for more sync primitives
extra: Add an intrusive MPSC to be used soon
std: Hardcode pthread constants and structures

This allows for easier static initialization of a pthread mutex, although the
windows mutexes still sadly suffer.

Note that this commit removes the clone() method from a mutex because it no
longer makes sense for pthreads mutexes. This also removes the Once type for
now, but it'll get added back shortly.
extra: Introduce a new efficient Mutex type

There are no current consumers of this mutex, but rather more will be added
soon.
extra: Re-add the Once primitve to extra::sync

This originally lived in std::unstable::mutex, but now it has a new home (and a
more proper one).
std: Remove try_send_deferred plus all fallout

Now that extra::sync primitives are built on a proper mutex instead of a
pthreads one, there's no longer any use for this function.