Consider moving back to serde::Serialize

[obmarg]

Consider moving back to serde::Serialize

[obmarg]

Sticking this here just as a note, found this which would let us support more than just JSON: https://docs.rs/serde-value/0.7.0/serde_value/enum.Value.html

[obmarg]

Investigated this and remembered the reason(s) I moved away from serde::Serialize in the first place:

1. We can't easily derive it for InputObjects or Enums as I've not found a way to add additional derives from a derive.
2. We can't ask the user to derive it as we need to rename fields etc (the user could do this, though then we'd need to parse the serde renames so we knew what fields things were.
3. serde::Serialize is not object safe so my approach of Boxing a SerializableArgument would need reworked. Could look into erased_serde (or whatever it's called) but not worth it right now.

Although it might be worth looking into using serde_value rather than serde_json - for now I'm just going to close this and focus on more important bits of the API.