simplify shootout-reverse-complement.rs

Simpler, safer and shorter, in the same spirit of the current version, and the
same performances.

@mahkoh please review, I think I didn't change any performances related thing.
AFAIK, yes. this is an adaptation of @alexcrichton parallel function on `shootout-spectralnorm.rs`.  He must know better than me if this function is really safe.