Tree type and methods

[dfellis]

With Alan remaining loop-less in both types and code, a Tree type to safely represent such structures is necessary. We can probably crib most of this from Alan v0.1

[dfellis]

The remaining pieces for the Tree type:

• subtree to take a Node{T} and produce a new Tree{T} with that node as the root and only include the child nodes
• addChild that takes a Node{T} to make a child from it. As it is a tree and not a Graph, this will be implemented with subtree under-the-hood.
• prune to remove a Node{T} and all of its children from a Tree{T}
• delete to remove a Node{T} and attach its children to its own parent node.