- Introduced enum type `thread_state_t` to replace preprocessor macros
- Moved thread states to `sched.h` for two reasons:
a) Because of the interdependencies of `sched.h` and `thread.h` keeping it in
`thread.h` would result in ugly code.
b) Theses thread states are defined from the schedulers point of view, so it
actually makes senses to have it defined there
These headers do not provide full stl functionality,
but a small subset:
* thread and this_thread
* condition_variable (some timed functions are missing)
* mutex, lock_guard and unique_lock
