Add Buckets

[SabrinaJewson]

Split off from #31. The notes from #31 (comment) still apply.

(Right… I need to fix merge conflicts. I will do so soon.)

[SabrinaJewson]

Alright, fixed the conflicts.

[ibraheemdev]

I left a couple of (hopefully minor) comments, otherwise this looks good. I appreciate the thoroughness here.

Is the MSRV bump absolutely necessary, or are there ways to work around the features being used? 1.81 is quite a big jump, but I wouldn't consider it a breaking change.

[SabrinaJewson]

So re MSRV concerns, the currently used features that are nontrivial to replace are:

• assert_unchecked (inside Index::new_unchecked): 1.81.0
• const Option::unwrap, three times: 1.83.0

Polyfilling the later (either inline or with a helper) is possible, yes, so I could implement that. The former is not possible to polyfill, but I could just panic in new_unchecked instead. Thoughts?

(also, thoughts on boxcar::Iter being moved to boxcar::vec::Iter – should I reexport it?)

[ibraheemdev]

Can you use hint::unreachable_unchecked for the former?

Yeah, we can re-export until the next breaking change.

[SabrinaJewson]

…You’re right, I’m embarrassed I forgot unreachable_unchecked exists. Made the changes. Now it’s just complaining that I made the re-exports #[doc(hidden)]… it kind of makes sense to me as a way to soft-deprecate it, but I guess I can just not have it be #[doc(hidden)] as well lol.

[ibraheemdev]

This looks great, thanks! I'll cut a release soon.