* move pfx out of variant which avoids pointless field type panic checks
and copies on access
* make `VertexRef` a non-inheritable object which reduces its memory
footprint and simplifies its use - it's also unclear from a semantic
point of view why inheritance makes sense for storing keys
* Implement partial trees
why:
This is currently needed for unit tests to pre-load the database
with test data similar to `proof` node pre-load.
The basic features for `snap-sync` boundary proofs are available
as well for future use. What is missing is the final proof verification
and a complete storage data load/merge function (stub is available.)
* Cosmetics, clean up
Jacek Sieka
Jordan Hrycaj