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.
In the Reader doc, read can return Ok(0). Theoretically, using push may eat CPU without limit. In practice, as read should avoir returning Ok(0), it should work as expected.

Then, I don't know if we should be correct using push_at_least or potentially faster using push. As I can't see any performance changes I personnally prefere correctness.