* Keep vertex ID generator state with each db-layer
why:
The vertex ID generator state is part of the difference to the below
layer
* Move otherwise unused source to test directory
* Add Merkle hash generator
also:
* Verification facility for debugging
* Empty Merkle key hashes encoded as `EMPTY_ROOT_HASH`
details:
1. Merging a leaf vertex merges a `Patricia Trie` path (while
adding/modiying vertices) and adds a leaf node with payload
2. Merging a Merkel node merges a single vertex to the `Patricia Trie`
and registers merkel hashes
3. Action 2 can be used before action 1 in order to construct a
Merkel proof as required for handling `snap/1` data.
4. Unit tests show that action 3 is benign for now :)