Fixing bugs in C implementation.

- Enabled global object lists and fixed a couple related bugs.
- Got virtual thread entry point working for Ar::Thread, though it could
  be made simpler.
- Added a reference pointer field to ar_thread_t, used to point back
  to the Ar::Thread instance.
- Made Ar::Thread::getCurrent() use the new ref field to get the Thread
  instance from an ar_thread_t.
- A few other minor changes.
- Added an item to the todo list.
Renamed public includes for Argon.

- Added an argon.h header that include both the C and C++ APIs.
- Wrapped C++ API so it will be ignored by a C compiler.
- Updated include directives in test code.