Add support for System serialization

[shaoner]

Proposal

The idea is to add support for serde -no-std- serialize / deserialize in System and recursively to cpu, bus, rom, mbc, ppu, joypad, timer, serial.

This would let users to provide whatever serialization they want (for ex: msgpack)

Notes:

• We also need to add some kind of rom hash signature to ensure the rom matches the save file
• Rom storage should not be serialized
• We may need to reset some registers in the process like the ones in joypad

Motivation and context

Let users eventually save & restore their game