Introduce a class for calculating Damerau-Levenshtein edit distances and strings

Summary:
This helps us attack three problems:

• We currently can't give string transpositions a cost when evaluating British spellings for arc, so "alnd" has equal edit distances to "amend" and "land".
• The intraline differ isn't UTF-8 aware, and the algorithm is character oriented.
• ArcanistDiffUtils::buildLevenshteinDifferenceString() is a tangled mess of disgusting garbage.

Introduce a better-factored implementation of Damerau-Levenshtein with options to price trnspositions and build edit strings.

Test Plan: Unit tests.

Reviewers: vrana

Reviewed By: vrana

CC: aran

Maniphest Tasks: T2379

Differential Revision: https://secure.phabricator.com/D5032