Support extend self by prototype rb

[pocke]

This pull request adds support for extend self by prototype rb.

For example, module M; extend self; end will be converted to module M; extend ::M; end. self that is in arguments of extend is converted to the module name in the current scope.

It converts self to an absolute type name. It means it always generates a full path as an argument of extend.
Because a relative path can be incorrect.
For example:

module M
  module M
  end

  # In this context, `extend M` means `extend ::M::M`,
  # so it should be converted to `extend ::M`.
  extend self
end

---

it can resolve include self with the same technique, but I didn't implement it because of meaningless.
include self raises an error, so actually no one uses this.

$ ruby -e module M; include self; end
-e:1:in `append_features': cyclic include detected (ArgumentError)
	from -e:1:in `include'
	from -e:1:in `<module:M>'
	from -e:1:in `<main>'

[soutaro]

💯