Make HashDB generic (#8739)

The `patricia_trie` crate is generic over the hasher (by way of HashDB) and node encoding scheme. Adds a new `patricia_trie_ethereum` crate with concrete impls for Keccak/RLP.

ethcore/sync/src/chain/supplier.rs

@@ -22,6 +22,7 @@ use network::{self, PeerId};
 use parking_lot::RwLock;
 use rlp::{Rlp, RlpStream};
 use std::cmp;
+
 use sync_io::SyncIo;
 
 use super::{

ethcore/sync/src/lib.rs

@@ -30,6 +30,7 @@ extern crate ethcore_transaction as transaction;
 extern crate ethcore;
 extern crate ethereum_types;
 extern crate env_logger;
+extern crate hashdb;
 extern crate plain_hasher;
 extern crate rand;
 extern crate semver;
@@ -38,6 +39,7 @@ extern crate smallvec;
 extern crate rlp;
 extern crate ipnetwork;
 extern crate keccak_hash as hash;
+extern crate keccak_hasher;
 extern crate triehash;
 extern crate kvdb;