demos: actors: A couple of simple actor model demos.

demos: actors: A couple of simple actor model demos.

* demos/actors/simplest-possible.scm: New file.
  Simplest possible demo: Two actors say hello to each other.
* demos/actors/botherbotherbother.scm: New file.
  A little bit more complex; multiple actors interact with each
  other.  Ensures that actors don't clobber each other unexpectedly.