Derive löb induction from later introduction and guarded fixpoints

See merge request FP/iris-coq!145

- Make `flöb_pre` and `flöb` local to the proof.
- The metavariables `Ψ` are used for predicates, so use a `Q` here.

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

proofmode: make it possible for class_instances to use the proofmode

See merge request FP/iris-coq!142

Let `done` and friends fail when the proofmode goal is an evar.

See merge request FP/iris-coq!140

This fixes issue #182.

  Also, remove the inconsistency that `wp_expr_eval` succeeds on a goal
  that is not a WP.