Add `with_route` methods to `IntoPicker` and `PickerPattern` for
setting the route type explicitly when needed, instead of carrying
it as a generic parameter on the trait itself.

Introduce `or_route` and a `PickerResult` type that carries an optional
route value.  When a flag is absent, the closure given to `or_route` is
evaluated and stored in `error_route`, allowing the caller to exit early
without a default value.

`PickerResult` struct