Support singleton class definition

[soutaro]

• Singleton class support in a class definition
• Singleton class support for an instance (but what should we do for this???)

# Singleton class definition in a class definition

class WithSingleton
  class <<self
    def open
      yield new()
    end
  end
end

# Singleton class definition for an instance

x = Object.new
class <<x
  def foo
    "foo!"
  end
end