Implement an analog of map_instance_to_supertype() for protocol supertypes

[ilevkivskyi]

In certain situation (like e.g. when dealing with *args) we want to know what are the type arguments of a generic protocol that a given type implements. Currently this is done in ad-hoc way in few places, but it would be great to have a dedicated helper.

This will also help with some advanced issues like #3829