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.
observed.
This is a prerequisite for async on_attach fairings. 'Rocket' is now a
builder wrapper around the 'Manifest' type, with operations being
applied when needed by 'launch()', 'Client::new()', or 'inspect()'.
'inspect()' returns an '&Manifest', which now provides the methods that
could be called on an '&Rocket'.