auto merge of #4922 : jbclements/rust/add-deriving-eq-to-asts, r=catamorphism

auto merge of #4922 : jbclements/rust/add-deriving-eq-to-asts, r=catamorphism

r?

Apply deriving_eq to the data structures in ast.rs, and get rid of the custom definitions of eq that were everywhere. resulting ast.rs is about 400 lines shorter.

Also: add a few test cases and a bunch of comments.

Also: change ast_ty_to_ty_cache to use node ids rather than ast::ty's. I believe this was a suggestion related to my changes, and it appears to pass all tests.

Also: tiny doc fix, remove references to crate keywords.