Simplify definition of is_unstamped_ty further.

[Blaisorblade]

Try modifying is_unstamped_ty to just use atomic_path_root, and drop is_unstamped_* on other syntax. That should also let us drop the free-variable count from it.

However, for examples, we still need an alternative inductive definition for nclosed on types (used for type members); but that should not be needed for path substitution.