Improve shootout-reverse-complement

Lots of unsafe code and lots of branches removed. Also multithreaded.

Rust old: 1.208 seconds
Rust new: 0.761 seconds
C: 0.632 seconds
I think that this parallelisation can be much more idiomatic and clean using a variant of the `parallel` function of spectral-norm using `split_mut` iterator or a reimplementation of such an iterator using your `memchr` function.