Install C header files.
Merge pull request #122 from yallop/install-headers

Install C header files.
Add a function ctypes_copy_bytes for building GC-managed blocks.
Add ctypes.stubs subpackage.
Add a module for validating and printing paths.
Generation of 'external' declarations from types.
Add bigarray support for external declarations.
Basic C stub generation support.
Basic printer for generated ML code.
Handle external names in generated ML stubs.
Add interface for ml_stubs module.
Generate wrapper functions for ml stubs.

Pass projected raw pointers to C.
Disable 'noalloc' for the moment.

We need an interface for specifying interaction with callbacks.
Disable the "float" attribute for the moment.
Generate byte code stub, passing arguments using argc/argv.
Give views and structured types an identity.
More work on view stub generation.
Extend registry to structured types.
Stub generation tests.
Test for calling atomic builtins via stub generation.