Use the derive-visitor crate to replace our hand-written visitors

[Nadrieril]

This uses the clever derive_visitor crate instead of our handwritten visitors. I managed to replace all of our visitor needs with it.

This PR is more readable commit-by-commit. Fixes #249.

[Nadrieril]

I'm a bit worried about the risk of stack overflow given that derive-visitor doesn't use ensure_sufficient_stack. I'm working on #122 to remove the main source of runaway recursion.

[sonmarcho]

That's great!
If we have a stack-overflow issue with one instantiation of the visitors, we can manually override some of its (well-chosen) functions to use ensure_sufficient_stack, right?

[Nadrieril]

Oh indeed, you're right! Let's merge then