Treat sealing empty structs or unions as an error.
Add the 'camlint' basic type.

Values of camlint appear as OCaml ints and have the same storage requirements
as value.  The main (and perhaps only) use is for exposing Bigarray.int_elt.
sizeof (nativeint) is now sizeof (void*), not sizeof (int).
Merge pull request #34 from yallop/nativeint-size

sizeof(nativeint) should be sizeof(void*), not sizeof(int)