add some assertions to track down a potential problem in default methods.
Add default impleentations for visitor trait methods when beyond stage0
since current snapshot compiler has bugs with default methods.
checkpoint attempt to add trait-based ast_map while keeping closure-based ast_map simulteanously.
Refactoring two E, vt params to one (E, vt) param, analogous to that of commit ecef9ad75adb929521e3e6a3cd2137964f7cd36c.
Added Visitor-trait compatible walk_crate entry point.
Try using walk_crate with ViaFns visitor.
Switch accept method to use Visitor-trait rather than ViaFns.
Switch from visiting ViaFns to visiting via impl Visitor for Ctx.