* chess-pos.el (chess-pos-en-passant, chess-pos-status)

* chess-pos.el (chess-pos-en-passant, chess-pos-status)
(chess-pos-side-to-move, chess-pos-annotations)
(chess-pos-preceding-ply): Enable use as generalized variables.
(chess-pos-p): New function.

* chess-ply.el (chess-ply-pos, chess-ply-changes)
(chess-ply-keyword): Enable use as generalized variables.
(chess-ply-castling-changes): Convert to using
`chess-next-index'.

* chess-polyglot.el (chess-polyglot-pos-to-key): Use logxor to invert
rank instead of two loops to keep rank/file apart.