Include key suffix in leaf's hash.

  This changes quite a few things and make the proof verification
  slightly more complicated, but overall remains manageable. This change
  is however necessary to prevent one from including entire sub-trees in
  one go by making look like a _value_; thus resulting in multiple trees
  with the same root hash.

  So for example, one could construct the following tree:

  ```
  ╔═══════════════════════════════════════════════════════════════════╗
  ║ #f6ee5ad5391e966a7e0f24465d98ab5df2596e5987f189b67479d271b6e938a7 ║
  ╚═══════════════════════════════════════════════════════════════════╝
   ┌─ 09ad7..[55 digits]..19d9 → apple
   └─ 177ca..[55 digits]..98a3 → <some gibberish value>
  ```

  Which is in fact, a way of disguising the following tree:

  ```
  ╔═══════════════════════════════════════════════════════════════════╗
  ║ #f6ee5ad5391e966a7e0f24465d98ab5df2596e5987f189b67479d271b6e938a7 ║
  ╚═══════════════════════════════════════════════════════════════════╝
   ┌─ 09ad7..[55 digits]..19d9 → apple
   └─ f #177cab65ae5d
      ├─ 3 #9a4b2591979d
      │  ├─ 2e49b..[53 digits]..9728 → pomegranate
      │  └─ 7d2a6..[53 digits]..d578 → kiwi
      └─ a3c8d..[54 digits]..52ff → plum
  ```

  The value is just chosen to be the pre-image of the 'f' node. However,
  this completely breaks the logic and guarantee that comes with
  insert/delete as now, one is able to add or remove entire parts of the
  tree in one go.

  The fix: includes the remaining key path in the leaf's hash, all the
  way up to the root.

  By including the remaining path (or suffix) in the final leaf, the
  trie is made completely tampered proof as the root now fully depends
  on not only the elements present in the tree, but also the structure
  of the tree itself. So, the root tree effectively depends on the
  number of branches, and their positions. So a situation like the above
  is no longer possible; the first tree would yield a different root
  hash and the proof would be invalid against the second tree.

  NOTE: The on-chain implementation, as well as the proof serialisation
  format must be adjusted.