Implement const

Cython 0.18 support the const keyword, hence it makes sense
to output it. This commit also processes the volatile, but it
isn't used for the final output.
const for pointers

This commit needs Cython > 0.18 as support for
const pointers was just recently added with commit
5e1497371ee70fffb2deb6c7dab136cf6684f2df [1].

[1] https://github.com/cython/cython/commit/5e1497371ee70fffb2deb6c7dab136cf6684f2df