Modifications for better cross-project use of HashDB/Patricia Trie

[rphmeier]

cc @debris

HashDB interface currently uses local primitive types, while with the primitives crate approach local types can be used. Also the assumption on keccak is not likely to remain strong.

A modification of the interface from

trait HashDB {
    fn insert(&mut self, value: Vec<u8>) -> H256; 

    ....
}

trait HashFunction {
    type Out: Debug + PartialEq + Eq + Clone + Copy + Hash + Send + Sync;

    fn hash(x: &[u8]) -> Self::Out;
}

trait HashDB<F: HashFunction> {
    fn insert(&mut self, value: Vec<u8>) -> F::Out;

    ....
}

would be a definite improvement and lead to the crate carrying around fewer dependencies.

---

One problem is the special-casing of many known hashes like NULL_RLP, RLP_EMPTY, etc. in the patricia trie or in memory db. How can we handle the special cases correctly without re-hashing or doing extra work on each hash?

[rphmeier]

Trait aliases makes this even better, but a local trait KeccakDB: HashDB<Keccak256> {} with a blanket impl would be fine too.