func RatioForStrings(source []rune, target []rune, op Options) float64 {
	matrix := MatrixForStrings(source, target, op)
	return RatioForMatrix(matrix)
}
func RatioForStrings(source []rune, target []rune, op Options) float64 {
	matrix := MatrixForStrings(source, target, op)
	return RatioForMatrix(matrix)
}
// RatioForStrings returns the Levenshtein ratio for the given strings. The
// ratio is computed as follows:
//
//     (sourceLength + targetLength - distance) / (sourceLength + targetLength)