This is part of the `App` rename.
Previously, I was concerned about not being able to deprecate
For backwards compatibility, we still expose the `IntoApp` name.
No good solution for transitioning the trate name, unfortnately, since
we can't mark `use`s as deprecated (we can, it just does nothing).
I got rid of the `into` prefix because that implies a `self` parameter
that doesn't exist.
