Create `Tree` struct to search for tree counterexamples

[ariasanovsky]

Several of the counterexamples from the Wagner paper were trees-like, or led to tree counterexample constructions. It's possible that the cross entropy method from the paper is more adept at searching for sparse graphs.

To support sparse (tree) constructions:

• [x] generate trees from Pruefer codes
• [x] impl ShortForm either to g6, s6, or Pruefer code with an analogous deserialization in python
• [x] impl State for Tree<N>
• [x] test Conjectures 2.1 and 2.3 using trees