MAINT split up `player` into `humanPlayer` and `computerPlayer`

[eigenfoo]

move would then be implemented as a pure virtual function in a player abstract base class, to be defined in humanPlayer and computerPlayer. This would certainly be a cleaner way to organize my code, but it's not immediately apparent to me how to go about doing this.

[eigenfoo]

It is impossible to allocate memory to an abstract base class. What is possible, however, is to declare pointers to an abstract base class, and then delay the specification of computerPlayer or humanPlayer to later.

This seems to be the way this problem is dealt with, but it's certainly not elegant.