Fix up cfg attributes to work on an xor basis.
Previously, the cfg(any()) attributes would cause issues when
both native-tls and tls features were enabled. Now, https functions
and enum variants will only be created when tls xor native-tls are
enabled. Additionally, a compile error has been added for when
both tls and native-tls features are enabled.
This removes the necessity to take the result of Response::into_json and
having to convert it into a struct by using serde_json::from_value
This adds no new dependencies since serde_json already depends on serde.
Users of ureq will have to include `serde_derive` either by importing it
directly or by using serde with the `derive` feature, unless they want to
manually implement `Deserialize` on their structs.
