Handle size_t / ssize_t specially.

This makes it so that usages of size_t / ssize_t will get mapped
to the new C-FFI support for <C-size-t> and <C-ssize-t>.

Part of this patch treats size_t / ssize_t as new reserved words
that map to the correct pre-defined types. The other part adds
a special case to the typedef processing to skip any typedefs that
attempt to set the meaning of size_t / ssize_t, like this from the
OS X headers:

    typedef __darwin_size_t size_t;