lifted.freeVariables: Don't overload by type, assert equality

[marzipankaiser]

This changes lifted.freeVariables to compare variables by id only. It, however, asserts equality for the params, too (They should be consistent).

Note: This makes the following example from #225 crash:

effect Increment {
  def increment(): Unit
}
def main() = {
  try {

    def step(): Int = {
        def looper(): Int / Increment = 0
        looper()
    }

    step()
  } with Increment {
      def increment() = resume(())
  }
}

This is a good thing, as this makes a bug in PolymorphismBoxing obvious.

[b-studios]

Could you add the example as a test? This way we can avoid reintroducing the bug.