It supports parsing arguments and inserting custom strings into the help
message.

And complain if there is no place left to throw any stones.

We don't really need it, we don't have Game records with nested Board records
anymore. It only adds noise.

This allows them to display errors in the middle of input gathering (before
asking for input but after printing the board for example).

- PlayerType records allow pluggable player types (cli, gui, network, computer)
  that have to implement a series of functions.
- The game record has now a Player -> PlayerType map.
- The game loops (play) are now not coupled to the CLI anymore.
- The Action data type no longer exists - Maybe Position for move/quit and
  Player for switch.