differential insertion on hashed leaf nodes

[gballet]

Second experiment for addressing #229 (see #231) : the signature of Insert is changed so that when values are inserted, and if a node is a hashed leaf node (i.e. a HashedNode with stem != nil), then no value is actually inserted and its commitment is computed from the delta between the old value and the new one.

Although its semantics haven't changed yet, LeafNode is expected, in the future, to only be used to initialize an extension-and-leaf-node, and never for an update. The end goal is to only use it into InsertStemOrdered and discontinue both InsertOrdered and InsertStem (if possible, it might be difficult to achieve because of an out-of-order conversion process).

With this method, LeafNodes are never directly stored into the DB: values are read from the snapshot, and provide all the necessary information to update the tree without storing the serialized list of values. This makes the whole tree smaller both in RAM and on disk. It will also reduce IO during the conversion process (pending a modification to embed the hashed node in its parent InternalNode) and root commitment calculations (no full polynomial evaluation at each node)

TODO:

• also update the commitment of its parents
• fix TestInsertIntoHashedNode
• also implement it for StatelessNode
• rebase once the hashed leaf nodes are stored directly into their parent InternalNode for reduced I/O.
• add a fuzzer against a LeafNode tree

Out of scope:

• Removal of InsertStem and InsertOrdered
• diff-insertion of multiple values into the same leaf

[gballet]

This has bitrotted for a long time, and most of its ideas are now implemented as part of other PRs. Closing.