actors: mlambda, removing require-slot, GOOPS usage cleanup

mlambda is a new nice sugar for passing in arguments, so add that.
Also, various problems found with GOOPS hackery in Guile 2.2.
Cleaning up.

* 8sync/systems/actors.scm (require-slot): Removed.  Doesn't work with
  Guile 2.2.
  (<actor>): Updated to drop usage of require-slot.
  (mlambda): New procedure allowing automatically pulling out message
  stuff from action method definitions.
  (%expand-action-item): Use mlambda.
  (<hive>): Remove redundant slot definition back to hive.  The actor
  already does this, and not re-supplying the accessor breaks GOOPS
  in Guile 2.2.