Implement Subgame Solving

[zanussbaum]

This is the bulk of the computation for the real-time-search. Once this is implemented, we have a basic MVP.

[zanussbaum]

A good first starting point would be to implement this for Leduc Poker

[zanussbaum]

We also need to figure out a way to denote when a leaf node is reached. Should we build another tree or should we just keep track of the possible public states of a leaf node and check if we reach that state?