Subtree Support needs to be Completed & Tested

[bytemaster]

A database value can either be "data" or a root of a "subtree"
Binary nodes store inline data or pointer to larger value node.
Value nodes have a flag that indicates if they store a subtree.

It should be possible for binary nodes to have a flag to store subtrees directly in inline data because the value_type field is 4 bits and only 2 types are in use.

This will require testing the reference count recovery algorithm which looks for every subtree as a clue to increment the reference count.

This is needed before we can move psibase to arbtrie.