Replace RustcEncodable with Serde for savestate serialization

[simias]

At the time the savestate architecture was first implemented Serde was not yet stable. Since then it's become the de-facto serialization library for Rust, superseeding the hacky RustcEncodable/RustcEncodable.

While there's no emergency it'd be nice to move to Serde now that it's stabilized. It shouldn't be too difficult but it's quite a big change which impacts both rustation and rustation-libretro. In particular the current binary serializer would have to be ported (or rewritten). I'm not sure how straightforward that would be.