Consider using `ForeignPtr` with finalizers for memory managed in C

[jorisdral]

There are some C objects of which the memory is managed in C. We could use ForeignPtr rather than Ptr internally to emphasise that this memory is foreign-managed. Moreover, we could then attach finalisers to the ForeignPtr to ensure that the foreign-managed memory is freed when the corresponding ForeignPtr is garbage collected.

Example: CXToken structs are allocated by the libclang C library and they should be freed using clang_disposeTokens.