slug.go
func SubstituteRune(s string, sub map[rune]string) string {
	var buf bytes.Buffer
	for _, c := range s {
		if d, ok := sub[c]; ok {
			buf.WriteString(d)
		} else {
			buf.WriteRune(c)
		}
	}
	return buf.String()
}
func SubstituteRune(s string, sub map[rune]string) string {
	var buf bytes.Buffer
	for _, c := range s {
		if d, ok := sub[c]; ok {
			buf.WriteString(d)
		} else {
			buf.WriteRune(c)
		}
	}
	return buf.String()
}
// SubstituteRune substitutes string chars with provided rune
// substitution map. One pass.