Better support for singleton types in modules

Motivating example:

module SoftDeletable : ActiveRecord::Base
  module ClassMethods : singleton(ActiveRecord::Base)    # New syntax!
  end
end

Here, the implementation of SoftDeletable would use ActiveRecord::Base methods from included hook:

module SoftDeletable
  included do
    scope :deleted, ...
  end
end

1. Allow having singleton types in module self type constraints
2. Let singleton(SoftDeletable) < singleton(ActiveRecord::Base) (but what to do at the module context?)

Start from the motivating RBS module example and the requested module self type constraints. Determine the intended behavior for singleton(SoftDeletable) being a subtype of singleton(ActiveRecord::Base), including the module-context question; done means the syntax and subtyping rules are specified and covered by tests.