Auto merge of #123561 - saethlin:str-unchecked-sub-index, r=scottmcm

Use unchecked_sub in str indexing

rust-lang/rust#108763 applied this logic to indexing for slices, but of course `str` has its own separate impl.

Found this by skimming over the codegen for https://github.com/oxidecomputer/hubris/; their dist builds enable overflow checks so the lack of `unchecked_sub` was producing an impossible-to-hit overflow check and also inhibiting some inlining.

r? scottmcm

Author: bors