benchmark using difflib's SequenceMatcher

[thatbudakguy]

see the docs on Differ, which offer:

The basic Ratcliff-Obershelp algorithm is cubic time in the worst case and quadratic time in the expected case. SequenceMatcher is quadratic time for the worst case and has expected-case behavior dependent in a complicated way on how many elements the sequences have in common; best case time is linear.

if performance for expected inputs (i.e. the kanripo corpus) is asymptotically linear, it might be worth swapping out the index-based method to instead diff the two doc's sets of ngrams, as in text-matcher.