Draft ctypes blog post.

I've updated src/data.ml with the title and preferred URL (835f99f
<https://github.com/yallop/mirage-www/commit/835f99f47bfaf652e8fe668143c50dc5866f820d>).
I'd prefer "Modular foreign function bindings", but don't mind "Introducing
ctypes" if there's a strong reason to switch.