Method get_mut

publication/code/chapter06/hashmap.rs:170–173

(&mut self, key: usize)

    }

    fn get_mut(&mut self, key: usize) -> Option<&mut T> {
        let curr = self.get_pos(key);
        self.data.get_mut(curr)
    }

    fn contains(&self, key: usize) -> bool {
        if 0 == key { panic!("Error: key must > 0"); }