Enable unwrap_newtypes extension during serialization

[juntyr]

This PR fixes #284 and implements newtype unwrapping during serialization. E.g.

#[derive(Serialize)]
struct MyFloat(f64);

now serializes to 4.2 instead of MyFloat(4.2) or (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 This PR is quite minimal and just mirrors the deserialization implementation. A follow-up for unwrap_variant_newtypes is coming soon.

[juntyr]

@torkleyy Does this PR look good to you?

[torkleyy]

Thanks!