Unruly, Group: reference-count the 'immutable' array.

Unruly, Group: reference-count the 'immutable' array.

I noticed this during the bool trawl: both of these games have an
array of flags indicating which grid squares are immutable starting
clues, and copy it in every call to dup_game, which is completely
unnecessary because it doesn't change during play. So now each one
lives in a reference-counted structure, as per my usual practice in
similar cases elsewhere.