Implement basic game logic

Finish basic game logic.
Here's @rishisek 's comment on the logic, basically what we expect for a turn to do.

    /**
     * A single turn:
     * - Use
     * - Move
     * - Attack
     * - Buy
     *
     * DecisionSet becomes a compilation of these 4. At the end of a turn, decision_set is written to JSON.
     *
     * Turn {
     * players: [
     *  Player {
     *    // state
     *  }
     * ],
     * decision_sets: [
     *  DecisionSet {
     *    use, move, attack, buy
     *  }
     * ]
     * }
     */

    /*
    1. Create GameState
    2. For each turn of game,
        a. execute use, move, ...
        b. render turn
     */

Start by locating the existing GameState, Turn, Player, and DecisionSet entry points, then read how turns and decisions are currently represented. Compare the implementation with the issue's four actions—use, move, attack, and buy—and the stated JSON output. Done means the described game-state lifecycle, turn execution, rendering, and decision-set serialization are defined and working.