Beginning the process of converting internal representation of rank and file to integers.
Updated pieces to use integers 0-7 instead of string 1-8 and string a-h to represent rank and file.
Made first pass at updating tests to reflect the change from character representations of rank and file to integer representations.
Reworked Pawn tests in Piece module to reflect change of rank and file representation.
Tweaked Pawn; fixed wrong values for file and rank from which moving two spaces is allowed. Tests are now passing.
Added cache generation for knights.
Added test for rook's naive move generation.
Implemented move_to for rooks and king.
Implemented move_to test for Rook subclasses.
Knight is passing test for naive move cache generation.
Implemented test for Bishop naive cache generation.
Began updating game class to use 0-7 to represent both rank and file.