Add a `bumpalo::collections::ThinSlice` type

[QnnOkabayashi]

In my project, I've been using the Bump::alloc_slice_* family of methods a lot. Since the returned slices are freshly allocated, why not allow for a ThinSlice type that's just a single pointer into the Bump buffer that points to the length followed by that many elements? It would laid out a lot like ThinVec, but the data is owned by the Bump and with immutable length. You could then have it Deref/DerefMut to a regular slice as well, granting it all the methods and flexibility that slices come with.

For naming, we could just give each alloc_slice_* method a corresponding alloc_thin_slice_* method.

Pros

• Only a single word
• Maintains the same flexibility as a normal slice

Cons

• Would require some unsafe code to write
• Expands API surface
• How many people would find this useful besides me?

I'm happy to discuss or send in a PR for this if it sounds good to you.

[fitzgen]

I agree it is certainly possible to add these methods, but I'm not sure that the pros outweigh the cons here.

That said, I don't think there is any technical reason one couldn't build this on top of bumpalo in an external crate. I'd suggest investigating this route further.

Thanks!