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::{