Deprecate the serde module

[timvisee]

This deprecates the serde module and the PayloadConversionError.

Instead, this now recommends to use Payload::from_json_object(value) -> Option<Payload>. It seems to make more sense, and doesn't require a custom error type.

This also adds more useful conversion. For example, to convert form serde_json::Map (a JSON object) into Payload because this cannot fail.

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[JojiiOfficial]

I just found it being used in the snippets.
Might be a bit long to always type Payload::from_json_object(json!({ ... })).unwrap(). What do you think?

[timvisee]

Might be a bit long to always type Payload::from_json_object(json!({ ... })).unwrap(). What do you think?

Do you have another (name) suggestion to improve on this? A custom error like we had doesn't really follow the mantra we have with our general Error. Also, the error type does not add any more useful information than None does.

I'll merge this one now, but let's keep the above in mind and improve where we can.