Make patricia-trie usable from Polkadot (WAS: Extract patricia-trie to its own repo)

[rphmeier]

Along with various bits of generalization in it and its dependencies to make generic over hash function, hash type, and node encoding and decoding.

CC paritytech/polkadot#157

[dvdplm]

Tasks:

• make trait Hash generic over the hashing algorithm (investigate doing this in the hashdb crate, see Modifications for better cross-project use of HashDB/Patricia Trie #7019)
• make Node generic over hash; add a NodeCodec trait
• implement NodeCodec for rlp
• make sure encoding does not suffer performance hit; investigate using smallvec here
• port structures like TrieDB to be generic TrieDB<H: Hash, C: NodeCodec<H>>

@rphmeier looks about right to you? I'd like to do the extractions asap to avoid merge conflicts down the line as people might be making changes to the in-tree versions of the crates.

[rphmeier]

Looks about right. We might want to introduce a small crate just for the hash trait so other code (hashdb, triehash, etc.) can depend on it, and a separate crate for rlp-node-codec.