Rollup merge of #67725 - ssomers:into_key_slice_mut, r=RalfJung

Simplify into_key_slice_mut

Remove a rare and tiny but superfluous run-time check from into_key_slice_mut.

In #67459, I wrote that "`get_mut` [...] does visit `into_key_slice_mut`" and that was wrong. No function that operates on a map that (still) has a shared root ever dives into `into_key_slice_mut`.  So it's more clear to remove the (previously existing, and always incomplete) code it has for dealing with shared roots, as well as a petty performance improvement for those using exotically aligned key types.

~~Also, some testing of the `range` function initially added to #67686 but hardly related.~~

r? @RalfJung