Add fn #getPotentialKingAttacks and helper fn
#get-diagonal-positions.
Amend the fn #getPotentialKingAttacks to facilitate
its application across a set of positions.
Add constants for castling rights.
Amend parameter structure for the functions
#file-difference, #rank-difference, #kingside?, #kingward?, #castling-matcher;
add the fn #revoke-castling-right.
Add functions for the 'king' chessman type.
Add functions for the 'stepper' mixin.
Add specs for the 'stepper' mixin.
Add specs for the 'slider' mixin.
Add functions for the 'knight' chessman type.
Add functions for the 'rook' chessman type.