add #[allow(deprecated)] to derive implementations

[rcrisanti]

Allow deprecated in the Serialize/Deserialize derive implementations. This allows you to deprecate structs, enums, struct fields, or enum variants and not get compiler warnings/errors about use of deprecated thing.

Resolves #2195

[KarstenB]

I just encountered the same issue and was wondering how that could be implemented. While I am perfectly fine with the given PR, I thought about a more generic way. Like:

#[derive(Serialize, Deserialize)]
#[deprecated]
#[serde(allow = deprecated)]
struct bla {}

[rcrisanti]

@KarstenB that is definitely doable if that is the desired behavior. Personally I'm not sure I can think of a scenario where I do want to see these messages, so I'd at least be in favor of making the default not showing them, with some sort of option to turn them back on.

(For some justification of this, in rust itself they hide deprecation messages in derives, so in my opinion this would put serde more inline with expected behavior.)