Support save games

[bfirsh]

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Support built-in save games. High scores, save games, etc.

Not to be confused with #18 which is about snapshotting the entire emulator.

[bfirsh]

Sent to somebody interested in implementing this:

It’s called battery RAM, which you’ll see various references to in the code. I’d imagine it’d store it in local storage somewhere, perhaps keyed by a hash of the ROM.