Prepend the generic class's list of parameters onto the `impl`'s list of
deduced parameters, and add the `impl` to the impl scope enclosing the
class rather than adding it to the impl scope for the class body.
When matching an impl, deduce against both the type and the interface.
This incidentally slightly improves support for nested classes in generic classes, because they have some of the same properties as `impl`s nested in generic classes, but that's still a fair way from working for various unrelated reasons.
