Enable unwrap_variant_newtypes extension during serialization

[juntyr]

This PR is a follow-up to #333 and implements newtype variant unwrapping during serialization (i.e. the inverse of #319). E.g.

#[derive(Serialize)]
struct Inner { x: u32 }

#[derive(Serialize)]
enum MyEnum {
    Newtype(Inner),
    ...
}

now serializes to Newtype(x: 42) instead of Newtype(Inner(x: 4.2)) or Newtype((x: 4.2)). Enabling the extension also adds the extension enabling attribute to the top of the serialized RON.

• I've included my change in CHANGELOG.md

[juntyr]

@torkleyy As promised, here's my follow-up to #333 so that all implemented extensions now support both serialization and deserialization.

[torkleyy]

Thanks, LGTM!