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.
@mahkoh `vec.reserve_exact(cap + CHUNK);` give me results comparable with `vec.reserve_exact(2 * cap);`, i.e. as `Reader::read_to_end()` and slower than `vec.reserve_exact(mult * cap);`