Provides a cheapest possible (mainly in terms of memory usage) Mutex with the ConditionVariable bundled in.

Usage:

  class A
    include CheapLockable

    def do_exlusively
      cheap_synchronize { yield }
    end

    def wait_for_something
      cheap_synchronize do
        cheap_wait until resource_available?
        do_something
        cheap_broadcast # wake up others
      end
    end
  end