POSIX Threads support

The threads can be manipulated and synchronized using the classes `Thread`, `Mutex` and `Barrier`. This group also provides two optional modules with thread-safe collections: * `redef_collections` redefines existing collection to make them thread-safe. This incures a small overhead in all usage of the redefined collections. * `concurrent_collections` intro new thread-safe collections. Theses services are implemented using the POSIX threads. You can also use the `is threaded` annotation on methods, which makes them run on their own thread. Methods with self calls are not supported. A method or function annotated with `is threaded` has its return value changed during compilation. You will get a subclass of `Thread`, even if there wasn't a return value before. You can know if the threaded method is done with the `is_done` boolean from `Thread`. A call to the `join` method will block the execution until the threaded method is done, or immediatly return if it's already done. `join` will return an object typed with the orginial return type, or `null` if there wasn't.

Known limitations:

* Most services from the Nit library are not thread-safe. You must manage your own mutex to avoid conflicts on shared data. * FFI's global references are not thread-safe.

For more information:

* See: `man pthreads` * See: `examples/concurrent_array_and_barrier.nit` * See: ̀ examples/threaded_example.nit`