Implementation of Damerau-Levenshtein distance with transposition (also sometimes calls unrestricted Damerau-Levenshtein distance). It is the minimum number of operations needed to transform one string into the other, where an operation is defined as an insertion, deletion, or substitution of a single character, or a transposition of two adjacent characters. It does respect triangle inequality, and is thus a metric distance. This is not to be confused with the optimal string alignment distance, which is an extension where no substring can be edited more than once.