Threads

Intro the `threads` module with support for POSIX threads, mutex and barriers. Also provides 2 ways to have thread-safe collections: refining existing collections and sub-classing them.

TODO:
* Complete the collection support.
* Modifiy `print` so that printing the user content and the new line is (generally) atomic
* Add more examples/use in real programs
* Reinforce error support.
* Fix compatibility with FFI's global refs.
* Add, when needed, GC_THREADS to `gc_chooser.[c|h]`
* Do not include `gc.h` when not using libgc

ping @egagnon
Only GitHub supports the “task” syntax. Also you missed the space before `[`.

GitHub:

-[x] `ConcurrentArray`
- [x] `ConcurrentArray` (with the space)

Other Markdown parsers:

 -[x] `ConcurrentArray`
- &#x202F;[x] `ConcurrentArray` (with the space)