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.
The glibc allocator doesn't attempt to eliminate virtual memory fragmentation so it's able to use `mremap` as it exists today. However, it's significantly slower than just using `mmap`, `mremap` and `munmap` directly anyway.