Add a test for GameState::current_player().
The copy-constructor must not be marked as 'explicit'.
Initial implementation for GameStateGenerator. Only deals with Place actions.
Validate the copy-constructor inside tests.
Add implementation for generating successors by moving pieces.
Disallow copy and assign on GameStateGenerator.
Refactor the GameStateGenerator unit tests to be a TestWithParam.
Use the generator provided by the test fixture.
Add tests for moves that close mills.
Update valgrind suppressions.