A save api to supplement sync() in exports.

[io-tronic]

When using sync() to save data, it is required that the person playing the cartridge save it before quitting in order for the changes made by sync() to remain between sessions. This is a bit convoluted in the first place because if sync() is used when someone presses the save button in a game, they then have to save it again. But it really becomes a problem with exported versions of the build as those don't have a cartridge to save to and using sync() to save game data totally breaks down there. So having a function that would bring either bring up a save dialog for the player, or save in the background would be very useful.