implement counter with ahash

[chris-ha458]

I'm pretty sure this isnt the way you'd like it, but it does work.

If it is a feature that you might be looking for maybe we could adjust from here.

[coriolinus]

Agree that I don't really want to take this approach; using feature flags for this is convenient but very restrictive.

The approach more aligned to my style would be to add another generic parameter, with a default. So instead of being Counter<T, N = usize>, it would be a Counter<T, N = usize, B = HashMap<T, N>>.

To do that we'd need to design an appropriate trait (keeping in mind the map trait limitations!). We bypass many of those limitations because we don't want a fully generic map trait; the fundamental primitive operation we want is get_mut.

We could not implement this for BTreeMap because that would involve adding a bound T: Ord, which I would prefer not to do for backwards-compatibility reasons.

The benefit of all this work in this approach is that changing the backend becomes a user problem: they just need to implement the appropriate trait for the backing map of their choice, and then plug it in: type ACounter<T> = Counter<T, usize, ahash::HashMap<T, N>>;.

[coriolinus]

Looks like work has stopped here; I'm going to close this to tidy up the repo. If implementation resumes, feel free to reopen.