Add two BitBoard attributes to the struct that mark squares that should be marked
or highlighted.
Empty marked squares are shown with an asterisk (*). Marked squares with a piece
don't have any change of appearance. (Something I'm still thinking about.)
Highlighted squares are shown with the ANSI escape sequence for Reverse Video.
- ASCIIDisplay → format a type using ASCII only characters
- UnicodeDisplay → format a type using any Unicode characters
- FENDisplay → format a type for inclusion in a FEN string
