Auto merge of #109787 - scottmcm:index-slice, r=cjgillot

Add `IndexSlice` to go with `IndexVec`

Moves the methods that don't need full `IndexVec`-ness over to `IndexSlice`, and have `IndexVec` deref to `IndexSlice` so everything keeps working.

Doing this for later use in rust-lang/compiler-team#606, where I'm hitting a bunch of things that are just slices and thus there's no way to index with the `FieldIdx`.