make ffuzzy.h to include stddef.h
rewrite digest reader

Boost performance by rewriting digest reader engine.

Despite libffuzzy was designed to be fast, in some situations it was
slower than the original version. Now this is faster than libfuzzy
on almost all cases.

This commit also adds ffuzzy_digest structure which holds ssdeep
digest information after parsing.
make the interface C++-friendly
optimize edit_distn

This commit optimizes edit_distn function from assumption that
given strings are not empty.
release 1.1