Towards grudge typing

This improves many aspects of typing in arraycontext:

- It improves type checking (and consistency) in the traversals.
  - It allows scalars consistently.
  - It adds some overloads for traversal functions.
  - It adds `rec_map_container`, which is simpler (and easier to type).
- It makes array containers recognizable to the type checker.
  This works via a heuristic, by having `__array_ufunc__ == None`.
- It adds more types in the base fake numpy and shifts
  some implementation aspects there.