actors: Remove define-simple-actor, add define-actor.

define-simple-actor wasn't very useful, whereas define-actor is
moderately helpful.

* 8sync/actors.scm (define-simple-actor): Remove.
(define-actor): New macro.

* demos/actors/robotscanner.scm:
* demos/actors/simplest-possible.scm:
* tests/test-actors.scm: Convert to use define-simple-actor instead of
define-actor.