Add some features

[sonmarcho]

I need this for a PR on Aeneas' side

[sonmarcho]

I had to introduce several auxiliary visitors for definitions like global_decl, trait_decl, etc. because of a limitation of the visitor library: we can't derive visitors for a group of definitions containing structures with fields which have the same name (for instance global_decl and trait_decl both have a field named def_id, and if we try to derive one visitor for both definitions at once, we get a type error).

[sonmarcho]

Also, this PR introduces an LlbcAstUtils.iter_crate_with_span visitor which keeps track of the declaration id and of the last (most precise) encountered span when visiting declarations.