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.
It's not a performance bug in jemalloc or Rust and it's not a performance bug in the Linux kernel. I've proposed a new feature for `mremap` in the Linux kernel (`MREMAP_RETAIN`) to allow jemalloc to take advantage of it but there is no guarantee of that landing. I don't think you should include a FIXME for something that's not a bug and has no guarantee of ever being fixed.

You're free to use `mmap`, `mremap` and `munmap` directly which would be faster than working around the fact that you're doing massive copies by using a very large growth multiple. It's not possible for jemalloc to do this because `mremap` because it causes virtual memory fragmentation by unmapping the source.