// dropped from the byte slice with no replacement. The characters in s and the
// output are interpreted as UTF-8-encoded code points.
func Map(mapping func(r rune) rune, s []byte) []byte {
	// In the worst case, the slice can grow when mapped, making
	// things unpleasant. But it's so rare we barge in assuming it's
	// fine. It could also shrink but that falls out naturally.
	b := make([]byte, 0, len(s))