Make Coercible higher-kinded

This implements #8541. The changes are fully straight forward and work
nicely for the examples from the ticket; this is mostly due to the
existing code not checking for saturation and kindness.
Coercible: Do not try to unwrap undersaturated newtypes

otherwise we get a panic.
Explain higher-kinded Coerctions in Note [Coercible Instances]

(although there is not really a lot to explain, it seems.)