substr doesn't respects runes (utf8 characters)

[Klaus-Tockloth]

The current substr (substring) implementation does not respect runes (utf8 characters).

func main() {
	hello := "😞-Hello World-😞"
	fmt.Printf("substring = %s\n", substring(0, 5, hello))
	fmt.Printf("substringRune = %s\n", substringRune(0, 5, hello))
	fmt.Printf("substring = %s\n", substring(2, 7, hello))
	fmt.Printf("substringRune = %s\n", substringRune(2, 7, hello))
}

func substringRune(start, end int, s string) string {
	if start < 0 {
		return string([]rune(s)[:end])
	}
	if end < 0 || end > len(s) {
		return string([]rune(s)[start:])
	}
	return string([]rune(s)[start:end])
}

func substring(start, end int, s string) string {
	if start < 0 {
		return s[:end]
	}
	if end < 0 || end > len(s) {
		return s[start:]
	}
	return s[start:end]
}

Output
substring = 😞-
substringRune = 😞-Hel
substring = ??-He
substringRune = Hello

Remarks
substring(): current implementation
substringRune(): implementation that also works with runes