core: Made thread state an enum

- 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