Suggestion: additional expressive attributes

[anmonteiro]

Melange JSON could provide 2 features that I haven’t seen in other OCaml serialization libraries:

• [@skip_serialize_if Option.is_none]
• { … extra: (string * Json.t) list } (type TBD) that contains a grab bag of the remaining fields we didn’t parse

These are present in Rust’s serde and allow a lot more use cases to be expressed.

[andreypopp]

[@skip_serialize_if Option.is_none] should be an improved [@json.drop_default] really, now it only handles _ option but if we allow to specify equality (or maybe just use [%equal: t]? means we will depend on some other deriver) then it'd work, right?