wip: spa: deserialize: use Visitor pattern
Design inspired by Serde, see https://serde.rs/impl-deserializer.html Should make implementing a typed Value enum easier.
@ryuukyu : here is what I have so far with the Visitor pattern. Quite some boilerplate but it should make things more flexible. I still have to ponder how to handle array and structs properly. Does that match what you were expecting?