split fiber_manager in data part and free functions
use the free fm_<xyz> - functions
add volatile (not working)
some fixes
use TLS for pointer to fiber manager
use mutex + deque as ready-queue in example work-stealing
changes to support fier migration

- still not correct
some fixes
add precondition to awakened()
re-organize precondition for worker_fiber::resume()
fm_yield()/fm_join() do suspend active fiber

- in previous version active-fiber is stored in ready-queue
  even if active-fiber is in state RUNNING
-> triggers segmentation faults if fiber gets migrated
fix migrating a fiber between threads