bug#66908: Exposing more public nadvice API

[Philip Kaludercic]

I would like to ask if it would be possible to have a public and
"official" way to access information about advised functions, since all
the accessory functions defined by `oclosure-define' appear to be marked
as internal.

Specifically `advice-cd*r' would be useful, though it might be that we
(Visuwesh and I) are trying to do the wrong thing, since we want the
return value to get a more sensible response from `func-arity' -- and I
recall we had conversations about the complexity of this issue in the
past before.