RPC is dreamy

[aatifsyed]

• #3297 noted that our JSON approach was confusing
• #3339 introduced some standardisation with a HasLotusJson trait, and clarified the issues with the prior approach
• #3345 introduced the LotusJson<T> struct, and made the API cleaner and easier to implement
• #3390 removes the old json module altogether
• #3389 is outstanding
• #3383 is outstanding
• [ ] crate::ipld::json needs to be removed
• [ ] Consider splitting snapshots out of HasLotusJson
• [ ] #3638
  • Currently a bit reluctant to add macros to the codebase, and this would maybe only be useful for a handful of types at present
• [ ] Documentation needs clarifying on when it's acceptable to use #[serde(with = "crate::lotus_json")]
• [ ] trait RPCEndpoint needs design/implemention #3029
• [ ] removal of LotusJson<T> altogether