Ruby: Add type row for extends calls

[hmac]

A call to extend M adds all the instance methods of module M as class
methods to the receiver. For example:

module A
  def f; end
end

class B
  extend A
end

B.f # => targets A.f

To model this, we generate a typeModel row of the form

A, B!,

which can be interpreted as "the class B is an instance of the module
A". This is not strictly true, but means that class method calls on B
which are defined in A will be dispatched correctly.