480068c235
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 https://github.com/rust-lang/compiler-team/issues/606, where I'm hitting a bunch of things that are just slices and thus there's no way to index with the `FieldIdx`.