Implement SliceIndex for ByteStr

[thaliaarchi]

Implement Index and IndexMut for ByteStr in terms of SliceIndex. Implement it for the same types that &[u8] supports (a superset of those supported for &str, which does not have usize and ops::IndexRange).

At the same time, move compare and index traits to a separate file in the bstr module, to give it more space to grow as more functionality is added (e.g., iterators and string-like ops). Order the items in bstr/traits.rs similarly to str/traits.rs.

cc @joshtriplett

ByteStr/ByteString tracking issue: #134915

[rustbot]

r? @Mark-Simulacrum

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[joshtriplett]

LGTM, once it passes CI.

[thaliaarchi]

I needed to update snapshots for UI tests. Unfortunately, I think the diagnostics are now slightly worse. If these tests are on stable, they shouldn't be shown ByteStr.

I think it's also related to these impls being transitive: the index type implements SliceIndex<_>, then slice types implement Index for all types which implement SliceIndex<Self>. It would be better for suggestions to filter the index types to just those who implement SliceIndex<_> for the container.

[thaliaarchi]

@joshtriplett CI passed