Convert rune to int

An alternative to strconv.Atoi(string(n)) is int(n-'0') which should be safe after validating that n is a number with unicode.IsNumber()

if !unicode.IsNumber(runeValue) { return false } //this
runeNumber, _ := strconv.Atoi(string(runeValue)) // same as this runeNumber = int(runeValue - '0')