Exercise ideas: Game trees, Comma-Separated Trees, Collatz Trees

[Game trees](https://en.wikipedia.org/wiki/Game_tree) are trees that describe the possible choices in a turn-based game. The leaves are final states where either player has won, and each step up the tree describe the turn before where the other player (alternating at each level in the tree) has a number of choices corresponding to one sub-tree each.

An exercise would consist of describing a game between two persons (Alice and Bob?) in a story-like fashion, a (possibly illustrated?) description of game trees, and a task to 1) build the game tree between Alice and Bob for the given game, and 2) given a game tree, pick the best move.

The exercise can be unit-tested for small game trees and property-tested for larger game trees.

As for the specific game, popular choices are [Nim](https://en.wikipedia.org/wiki/Nim) and [Tic-tac-toe](https://en.wikipedia.org/wiki/Tic-tac-toe). We might also pick something else.

**Motivation:** In my opinion, the Haskell track lacks tree-based exercises. (Ping @petertseng)