Support serde flatten annotation for schema derivation and de/serialization

[Sufflope]

Fixes #247

[Kriskras99]

I think this is a good option for implementing support for #[serde(flatten)].
The one thing I don't like, is that we store the the flatten boolean in the custom attributes of RecordSchema. However, the only alternative I see, is adding another field to that struct which would be a breaking change.

@martin-g what do you think?

[martin-g]

@martin-g what do you think?

Honestly, I don't want to maintain anything Serde related :-|
Every time I tried to dive deep into Serde related code I see hacks and functionalities contradicting with each other.
I am not smart enough to understand why the serde crate is so popular...

[Kriskras99]

Serde is popular because it's really easy to use as a user. The complicated parts are hidden in serde itself and any crate implementing formats.

@Sufflope in the async work I'm also trying to make the serde part easier to maintain, I'll get back to this PR after the async work.