Rename ctypes-foreign project to ctypes-foreign-base.
Invert Foreign/Ffi/Closure_properties module dependencies with functors.
Split ctypes-foreign-base into threaded and unthreaded components.
Add a second version of ctypes.foreign that doesn't depend on threads.

Set up the META so that ocamlfind picks the appropriate version when
building a package that depends on ctypes-foreign.
Merge pull request #80 from yallop/optional-threading-support

Optional threading support
Fixes #38