Split invoker to invoker and invokerN

- Eliminate optional argument.
- Remove unnecessary check for `arguments.length`.
- Add unit tests.
> I don't have a good name for anti-tail, but I definitely agree that there's a long established practice of what head and tail mean that we wouldn't want to break.

Figured it might be something historical (but a quick search didn't turn anything up so figured I'd ask).

How about `front` for anti-tail?