Alpha-renamed top-level visit_* functions to walk_*.
(Motivation: Distinguish visit action and recursive traversal.)
Abstract over `&mut self` rather than over `@mut self`.
This required some acrobatics, notably the
`impl<E> Visitor<E> for @mut Visitor<E>`
and corresponding introduction of `@mut Visitor` and some local `let
mut` bindings.
Remove oldvisit reference.
Added default implementations for all of the Visitor trait methods.
Note that both `visit_expr_post` and `visit_ty` are no-op's by
default, just like they are in `oldvisit::default_visitor`.
Refactoring: extract logic to ease swapping visit for oldvisit (hopefully).
3d83010648