- delete std.Thread.Futex
- delete std.Thread.Mutex
- delete std.Thread.Semaphore
- delete std.Thread.Condition
- delete std.Thread.RwLock
- delete std.once

std.Thread.Mutex.Recursive remains... for now. it will be replaced with
a special purpose mechanism used only by panic logic.

std.Io.Threaded exposes mutexLock and mutexUnlock for the advanced case
when you need to call them directly.