Allow odd length prefix

[gpsanant]

allows for odd length prefixes.

still uses efficient logic for even length prefixes, uses less efficient logic for odd length prefixes.

disclaimer: I barely know rust

[nlordell]

Thanks for the contribution! One issue I see with the current implementation is that it allocates a Vec on the heap for each check. Ideally, it would be possible to do without this by comparing the bytes directly and potentially using a mask for the last nibble.

I'm not sure what the best way to structure the code for this would be.