actors: Implicit from-actor argument <-foo methods, and add rest of <-foo*.

* 8sync/actors.scm (<-, <-wait, <-reply, <-reply-wait): from-actor is
now implicitly supplied via %current-actor parameter.
(<-*, <-reply*): New procedures.
(<-wait*, <-reply-wait*): Now pull from-actor from #:actor.

* 8sync/actors.scm:
* 8sync/systems/irc.scm:
* demos/actors/botherbotherbother.scm:
* demos/actors/robotscanner.scm:
* demos/actors/simplest-possible.scm:
* demos/ircbot.scm:
* tests/test-actors.scm: Update all callers of <-foo procedures to drop
from-actor argument.

* doc/8sync-new-manual.org: Update documentation of <-foo procedures to
drop from-actor argument.