Index class/module references in RBS files

[soutaro]

#87 (comment)

Add constant references from type name references in RBS files.

class Foo < Array[String]
  include Bar[Integer]

  def foo: () -> Symbol
end

References to some type names like Array in super class and Bar in include are indexed since we need them for resolution. This issue is to add references from other type names in RBS, like type arguments and method type names. (String, Integer, and Symbol in the example.)