Implement compact encoding scheme for partial tries.

[jimpo]

As per module doc:

//! Compact encoding/decoding functions for partial Merkle-Patricia tries.
//!
//! A partial trie is a subset of the nodes in a complete trie, which can still be used to
//! perform authenticated lookups on a subset of keys. A naive encoding is the set of encoded nodes
//! in the partial trie. This, however, includes redundant hashes of other nodes in the partial
//! trie which could be computed directly. The compact encoding strips out all hash child
//! references to other nodes in the partial trie and replaces them with empty inline references,
//! indicating that the child reference is omitted. The nodes are then ordered in pre-order
//! traversal order so that the full nodes can be efficiently reconstructed recursively. Note that
//! hash references to nodes not in the partial trie are left intact. The compact encoding can be
//! expected to save roughly (n - 1) hashes in size where n is the number of nodes in the partial
//! trie.

[cheme]

Looks good.
I put a few question or suggestion or just general self comment (I will put them in resolve status).
I did play a bit with the pr on jimpo/trie-codec...cheme:try_compact_proof .
I still did not look at the 'Extension' management (I only focused on the branch only case).

[cheme]

I wonder if we could just debug_assert that (the function is only call with expect).