[Focus] Property based tests for Focus laws

[kenbot]

1. Applied/unapplied symmetry
   Focus[A](_.actions).command(a) == a.focus(_.actions).command

2. Internal/external operator symmetry
   Focus[A](_.actions.keyword) == Focus[A](_.actions).keyword

3. Internal/external composition symmetry
   Focus[A](_.action1.action2) == Focus[A](_.action1).andThen(Focus[B](_.action2))

4. Internal/external composition associativity
   Focus[A](_.action1.action2).andThen(Focus[C](_.action3)) == Focus[A](_.action1).andThen(Focus[B](_.action2.action3))