Actually implemented the serialise/deserialise functions in

midend.c. Also I've added an experimental front end in gtk.c only:
`Save' and `Load' options on the Game menu, which don't even show up
unless you define the magic environment variable
PUZZLES_EXPERIMENTAL_SAVE. Once I'm reasonably confident that the
whole edifice is plausibly stable, I'll take that out and turn it
into a supported feature (and also implement it in OS X and Windows
and write documentation).

[originally from svn r6030]

puzzles.h

@@ -186,6 +186,12 @@ char *midend_text_format(midend_data *me);
 char *midend_solve(midend_data *me);
 void midend_supersede_game_desc(midend_data *me, char *desc, char *privdesc);
 char *midend_rewrite_statusbar(midend_data *me, char *text);
+void midend_serialise(midend_data *me,
+                      void (*write)(void *ctx, void *buf, int len),
+                      void *wctx);
+char *midend_deserialise(midend_data *me,
+                         int (*read)(void *ctx, void *buf, int len),
+                         void *rctx);
 
 /*
  * malloc.c