This has the following nice benefits:
* The 'JsonValue' wrapper type is gone.
* 'Local{Request, Response}' natively support JSON/MessagePack.
* The 'json' and 'msgpack' limits are officially recognized.
* Soon, Rocket application will not require an explicit 'serde' dep.
This marks the beginning of the end of 'rocket_contrib'.
The connection guard type generated by `#[database]` no longer
implements `Deref` and `DerefMut`. Instead, it provides an `async fn
run()` that gives access to the underlying connection on a closure run
through `spawn_blocking()`.
Additionally moves most of the implementation of `#[database]` out
of generated code and into library code for better type-checking.