Fix self types in module

[soutaro]

1. Fix super_method calculation
2. Let self_types be classes
3. Assume Object is included in the self_types of modules if not specified

The 3 results in that we need to explicitly specify self_types if we want to include a module in Object

module M
end

class Object
  include M     # M implicitly assumes Object is the `self_type`, which causes a loop in ancestor chain.
end

The workaround is to specify BasicObject or an interface.

interface _WithSend
  def send: (Symbol) -> untyped
end

module M : _WithSend
end