Currently the packages have the following index functions:

func Index(s, sep []byte) int
func IndexAny(s []byte, chars string) int
func IndexByte(s []byte, c byte) int
func IndexFunc(s []byte, f func(r rune) bool) int
func IndexRune(s []byte, r rune) int

func LastIndex(s, sep []byte) int
func LastIndexAny(s []byte, chars string) int
func LastIndexFunc(s []byte, f func(r rune) bool) int

Searching for the last occurrence of a byte is quite common
for string parsing algorithms (e.g. find the last paren on a line).
Also addition of LastIndexByte makes the set more orthogonal.

Change-Id: Ida168849acacf8e78dd70c1354bef9eac5effafe
Reviewed-on: https://go-review.googlesource.com/9500Reviewed-by: Rob Pike <r@golang.org>