Implement S-Chess

Implement S-Chess

The value drops=2 in the gameState controls whether gating in pieces
from the holdings is possible, and implies holdings even if holdings=0.
New incarations of Cardinal and Marshall are added to the piece types,
under the names H and E. Gating has to be entered as promotion ('/' in
stead of '=' is allowed). King does no longer have to be the last piece
type; a macro PIECES has been added to indicate the number of piece types.