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

gmultiset RA

See merge request FP/iris-coq!138

The closing view shift's LHS mask is now universally quantified, which
makes it easier to execute the closing view shift.

This is a substitute for !136.

Fix regression in iGPS caused by eebe055b.

See merge request FP/iris-coq!120

As reported by @jjourdan: framing now no longer back tracks on whether
to strip laters or not. When framing below a later, we now only make it
strip laters of the head of the frame.