Is the `zero` instance member still required?

[coriolinus]

https://github.com/coriolinus/counter-rs/blob/9e217dcbe7f47d660428f11bb39504087a896474/src/lib.rs#L293-L294

That code was added in #9 (see discussion) and has never sat quite right. Can we work around it now? Leak exactly one zero item for each N? Bypass it some other way?