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 performance of huge reallocations on Linux is already better than other platforms. If `MREMAP_RETAIN` does get accepted upstream, then huge reallocations will be blazing on *on new Linux kernels* but that won't impact the performance on other platforms.

If you want to have code that's special-cased for Linux then you can already do that by calling `mmap`, `mremap` and `munmap`. It will be faster than what you're doing here because it will eliminate the huge copies rather than just making them less frequent.