This avoids the proofmode to be irritated by unrelated goals with a typeclass
type.

The BI interface is then instantiated using these laws.  In particular, this
shows that the rules we claim to be admissible actually are.

If the BI is not affine, this should not happen, as it may lead to
information loss.

This commit fixes issue #190.

These instances are the same as those for e.g. the later modality.

This fixes issue #187.

It now turns the goal into `P` and `<pers> Q`, which is dual to
`iDestruct`, which turns `P ∧ <pers> Q` into `P` and `□ Q`.

This follows the proof at https://en.wikipedia.org/wiki/L%C3%B6b's_theorem#Proof_of_L%C3%B6b's_theorem