Removed guff.
Implemented compare for Nat; fixed natLength

natLength was tail-recursive (it used an accumulator), but due to this,
it diverged for infinitely long lists. The new version is tail-recursive
modulo cons.
Removed guff from System.REPL.Command.
CLI.unknown now takes an infinite number of parameters.
Fixed the handling of empty command strings.

Empty command strings used to throw a NoActionFound exception or (if
that was removed) a user error. System.REPL.Command now now takes an
empty argument list to contain at least "" as the command name.