Introduce the concept of a `game_aux_info' structure. This is

constructed at the same time as an internally generated game seed,
so that it can preserve any interesting information known by the
program at generation time but not physically contained within the
text of the game seed itself. (Such as, for example, the solution.)
Currently not used for anything yet, but it will be.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@5729 cda61777-01e9-0310-a592-d414129be87e