Prepare serde branch for merge

[KieronJ]

Loading and saving states appears to be working, but with significant preliminary issues that make it a painful experience to actually use.

[KieronJ]

The following issues need to be fixed before this branch is ready to be merged:

• [x] Test loading and saving doesn't break input.
• [x] Test loading and saving won't cause weird issues with memory cards. Don't use save states whilst saving to memcards!
• [x] Switch to saving using a compressed binary format.
• [x] Reduce load/save time and size from ~15 seconds to 100 ms at most.
• [x] Reduce state size from ~14 MiB to something sensible.
• [x] Retrieve the current disc ID (e.g. SCUS-94154) and use different save states for each disc.