Method rehash

publication/code/chapter06/hashmap.rs:64–66

(&self, pos: usize)

    }

    fn rehash(&self, pos: usize) -> usize {
        (pos + 1) % self.cap
    }

    fn insert(&mut self, key: usize, value: T) {
        if 0 == key { panic!("Error: key must > 0"); }