fn map() -> [Decode<T>; 256] {
            let mut vec = Vec::with_capacity(256);
            for _ in 0 .. 256 {
                vec.push(Decode::Void);
            }
            vec.try_into().ok().unwrap()
        }
Creates a new map containing invalid values.
differential-dataflow/src/trace/implementations/huffman_container.rs