If the accessor introduces a binder, the first Coq-level intro pattern of `iInv`
is used for that binder unless the type of the binder is unit, in which case
`iInv` removes it completely.  Binders on the closing view shift are not (yet)
supported as they are harder to smoothly eliminate in the unit case.

New IntoAcc typeclass to decouple creating and elliminating accessors; ElimInv supports both with and without Hclose

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.