Avoid using instance/class types in modules

[soutaro]

The instance/class types are confusing in a module that is extended.

module Kernel
  def dup: () -> instance
end

class Object
  extend Kernel
end

What is the type of Object.dup? We might assume singleton(Object), but it is actually Object because instance type is an instance of the class declaration.

So, we should avoid using instance/class types in modules, especially when it is extended.