Serialize and Deserialize solve configurations to/from JSON

[bkushigian]

A postflop game is built from a CardConfig and an ActionTree. An ActionTree is built from a TreeConfig, a vec of added Lines, and a vec of removed Lines (Lines are just &[Action]).

We should make it easy to write tree/card configurations in JSON and serialize/deserialize these. Thus, if I wanted to invoke the solver from cli I can pass in the configurations from a JSON file.