Improve the translation of type declarations

[eqrion]

The current code for converting a Rust type to a C declaration is not correct and doesn't handle all the intricacies needed. It works for simpler things, like nested pointers, simple arrays, and simple function pointers, but I don't have confidence it works for arbitrary composition of those things.

Some broken things:

1. Functions that return function pointers
2. Function pointers that return function pointers