Adds a method to perform full-state snapshots by diffing against an empty state trie.
This replicates the functionality of `ipld-eth-state-snapshot`, so that code can use this as a library; see: cerc-io/ipld-eth-state-snapshot#1
Note that due to how incremental diffs are processed (updates are processed after the trie has been traversed) the iterator state doesn't fully capture the progress of the diff, so it's not currently feasible to state diffs this way. Full snapshots don't have to worry about updated accounts, so we can support them.
Co-authored-by: Thomas E Lackey <telackey@bozemanpass.com>
Reviewed-on: #15
