Node removal optimisation

[Totodore]

Currently when a node is removed from the tree there is no mechanism to check for optimisations.

Here is a non-exhaustive list of the possible things to do:

• Rewind the branch and check if nodes can be removed (only one child without value). Stop at the first non-removable node.
• Rewind the branch and check if nodes can be merged together (only one, non-wild child that doesn't have a value).