New file format

[Tuomas Lukka <lukka@xxxxxxxxxx>]

Ok, I've just checked in the changes: the new file format now works.
Activate it with "-cache" switch on command line. Any volunteer to make
this default and make it convert old spaces to this? SpaceVersion isn't
sufficient here.

We need to add a magic number to the beginning of these files still so
don't save anything in the new format yet: it'll change for a couple of
days with no upgrade path but after that we'll try to retain the upgrade
paths.

Related to this: the new code should be able to do undo as well, but it's
not yet hooked up to the user interface. Feel free to do it if you want:
stamps should go in after user-invoked ops (except of course undo/redo).

	Tuomas