Actually implemented the serialise/deserialise functions in

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]