Better %def-rec

%def-rec won't introduce any special void state anymore. Instead of prealocating an environment slot it will update the env part of the value being bound once it is evaluated. This change makes %def-rec postfix and makes it assume it is passed a combinator (operative flavour for now, there's no set-env! for applicative combinators defined yet). The %def-fin primitive wasn't needed anymore, so it was removed.