more zip versions

# version 1

    sub infix:<Z>(*@@args) {
        my @lists = @@args>>.list;
        gather {
            take @lists>>.shift while all(@lists);
        }
    }



# version 2

    # Assume Iterator has .next ("move to next element, return true/false")
    # and .value ("give me the element you're currently on") methods.
    # Then we have:

    sub infix:<Z>(*@@args) {
        my @iters = @@args>>.iterator;
        gather {
            while all(@iters>>.next) { take @iters>>.value; }
        }
    }


# version 3, using sentinel return from Iterator

    sub infix:<Z>(*@@args) {
        my @iters = @@args>>.iterator;
        gather {
            loop {
                my @results = @iters>>.get;
                last if any(@results) == SENTINEL;
                take @results;
            }
        }
    }

# version 4, using two-continuation .get2

    # assume $stream.get2(:on_end(&code)) does like so:
    #   case Just $value: return $value
    #   case End:         call code(), never return

    sub infix:<Z>(*@@args) {
        my @iters = @@args>>.iterator;
        gather {
            loop {
                my @results = @iters>>.get2(:on_end{ last; });
                take @results;
            }
        }
    }