Add overflow checking for `str::get` with inclusive ranges
Fixes https://github.com/rust-lang/rust/issues/42401
Two commits here:
1. The first makes `str::index` just call `SliceIndex<str>::index`. It's intended to have no behavior change, except where the two methods were inconsistent.
2. The second actually adds the overflow checking to `get(_mut)` (and tests for it)