Make the hashbrown dependency optional

[jplatte]

I was uncertain whether pull in iddqd for a project where it would be kinda useful, but potentially not worth the cost of the extra dependency. Pulling in hashbrown and rustc-hash definitely pushes it towards "not worth", but it seems like this would be easy to fix? I'm only interested in using IdOrdMap, and would be more than happy to submit a PR feature-gating all the hashmaps if you agree this is a good idea.

[sunshowers]

Thanks for the issue. So we do currently use a hashbrown hash table to map indexes to items, even with IdOrdMap (#22 (comment)) -- however, that is very much a shortcut that I used for an MVP, and more linear storage like a vector + a bitmap of initialized entries would likely be faster. If you'd like to implement that, I'd be all for it.

[jplatte]

Ah, that seems like a larger time investment than I came for ^^
I guess at least now the idea is approved for when I or somebody else wants to put in the necessary time :)