there's no need for the worker specialization here (isShutdown and shutdown are aliased to isJoined and join respectively), and the unstacking is not really desirable either as we previously learned with AsyncPool.
Despite the comments, there doesn't seem to be an obvious reason for these to be copy-pasted. Perhaps there was some legacy reason for this with legacy pthreads.
In fact, it looks likely that quit() will probably be able to be traitified too.
this has the most reliable behaviour, because some stuff like respect/validation likes to include files unconditionally which declare functions that are already declared, which breaks due to the lack of function autoloading.
