Move Cstubs_internals and FOREIGN from cstubs to ctypes

[yallop]

At present the interfaces used to write bindings that generate stubs (Cstubs.FOREIGN), the module that supports generated stubs (Cstubs_internals) and the functions that generate stubs (Cstubs.write_ml etc.) are all part of the same package (ctypes.stubs). Consequently, generated code depends on the ctypes.cstubs package, which sometimes causes problems (#519), since ctypes.cstubs depends on str.

This PR moves Cstubs.FOREIGN and Cstubs_internals from the ctypes.stubs package to the ctypes package, so that generated code no longer depends on ctypes.stubs. The change is backwards-compatible, so existing code should continue to work without any changes. However, if you'd like to eliminate the runtime dependency on ctypes.stubs then you can change the path of the FOREIGN interface in the Bindings functor to refer to Ctypes rather than Cstubs, as follows.

Existing way of writing bindings:

module Bindings(F: Cstubs.FOREIGN) = ...

Writing bindings without the runtime ctypes.stubs dependency:

module Bindings(F: Ctypes.FOREIGN) = ...

This PR addresses some of #528, but additional (and similar) work is still needed to move the TYPES interface from ctypes.stubs to ctypes.