* 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
* Extracted `test_tx.testTxMergeProofAndKvpList()` => separate file
* Fix serialiser
why:
Typo lead to duplicate rlp-encoded nodes in chain
* Remove cruft
* Implemnt portal proof nodes generators `partXxxTwig()`
* Add unit test for portal proof nodes generator `partAccountTwig()`
* Cosmetics
* Simplify serialiser return code format
* Fix proof generator for extension nodes
why:
Code was simply bonkers, not detected before the unit tests were
adapted to check for just this.
* Implemented portal proof nodes verifier `partUntwig()`
* Cosmetics
* Fix `testutp` cli poblem
