simplify shootout-reverse-complement.rs

Simpler, safer and shorter, in the same spirit of the current version, and the
same performances.
rephrase some comments according to remarks in the PR
auto merge of #18357 : TeXitoi/rust/simplify-reverse-complement, r=alexcrichton

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.