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.
If MREMAP_RETAIN lands then it will start using `mremap` for huge (>4M) allocations but there won't be an impact anywhere else. The improvement needs to happen in the Linux kernel, not jemalloc.