Add Method#super_method

Implements `Method#super_method` per https://bugs.ruby-lang.org/issues/9781

I'd like some extra review on this one, due to the RubyModule#== change I made. This change was made because `Module#==(IncludedModuleWrapper)` would always return false (even if the IncludedModuleWrapper instance wrapped the receiver), while `IncludedModuleWrapper#==(Module)` would return true. I'm not sure if this is the right change, but it *seems* to not break anything.

There is one outstanding stdlib test failure which is due to a difference in undef behavior in MRI and JRuby. I'm filing an issue for it.