bytes,internal/bytealg: add func bytealg.LastIndexRabinKarp

Also rename 'substr' to 'sep' in IndexRabinKarp for consistency.

Change-Id: Icc2ad1116aecaf002c8264daa2fa608306c9a88a
GitHub-Last-Rev: 1784b93f53d569991f86585f9011120ea26f193f
GitHub-Pull-Request: golang/go#63854
Reviewed-on: https://go-review.googlesource.com/c/go/+/538716


Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>