Merge branch 'coq-8.6' into 'master'

Coq 8.6

Once we make Coq 8.6 mandatory, we should merge this.

@robbertkrebbers I removed your `v8.6` branch (last commit: <https://gitlab.mpi-sws.org/FP/iris-coq/commit/b144f52f94c14ee5da1412b73a2d49981f9850b8>).  For once, it had the wrong name (you're not talking about Iris 8.6, are you? ;-); also, all it was doing was working around a bug in Coq that has since been fixed there.

See merge request !30