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.

This reverts commit 45162e5a.
We have to wait until ssreflect makes an 8.8-compatible release.