And make constants P for which we do not want of_val P to reduce Opaque.

through definitions.

This is more consistent with wp_cas_fail.

We may want to do the same for iIntros ? and iDestruct "H" as {?} "H",
but that requires more work. However, I do not think I want to rely on
names chosen this way.

We now give frame_here priority 0, so it is used immediately when an
evar occurs. This thus avoids loops in the presence of evars.

iSpecialize and iDestruct.

These tactics now all take an iTrm, which is a tuple consisting of
a.) a lemma or name of a hypotheses b.) arguments to instantiate
c.) a specialization pattern.

These tactics are superfluous:
- iPure H as pat => iDestruct H as pat
- iPersistent H => iSpecialize H "!"