lcs: Add C-based implementation for speed

This obviously makes the build platform-dependent. Probably doesn't work
on Windows. Patches accepted ;) The behaviour is still hidden behind the
--fuzzy option to annotate as there is still some lag. However, for a
rough idea, using the C implementation I could classify ~470
descriptions per second on a Lenovo X201s from a file containing 1575
annotated transactions.
README: Note requirement for a C compiler