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
```rust
match iter.find(|&(_, b)| b == b'\n') {
    Some((i, _)) => start = i + 1,
    None => break
}
```