actors: Switch over to using wrap-rmeta-slot for build-actions.

actors: Switch over to using wrap-rmeta-slot for build-actions.

* 8sync/rmeta-slot.scm (wrap-rmeta-slot): New variable.

* 8sync/actors.scm (build-actions): Switch from using make-rmeta-slot
to using wrap-rmeta-slot.

* doc/8sync.texi (Writing our own actors): Update documentation
to use #:init-thunk.

* 8sync/actors.scm (<actor>, define-actor, <hive>):
* 8sync/systems/irc.scm (<irc-bot>):
* demos/actors/robotscanner.scm (<warehouse-room>, <droid>):
* tests/test-actors.scm (<hi-on-init>):
* tests/test-rmeta-slot.scm (<kah-lassy>, <sub-lassy>): Update to
use #:init-thunk instead of #:init-value on actions slot.