Small optimisations to match.c

Hi Vince,

Thanks for an awesome tool!

Managed to decrease run time to ~75% of what it was. These optimisations basically just make the `score_sequence` and `find_best_match` faster by removing calls to `strlen`, removing (IMHO) redundant asserts and using `size_t`s to index arrays in inner loops.

Md5sums of output are the same, and the two tests pass.

Hope you find it helpful!

Cheers,
Kevin