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

Unfortunately, we currently have to keep the unicode-space hack in some places because Coq still complains about the notation otherwise

This approach is originally by Robbert

There are certainly more places this is useful, but let's start with this simple test

Fix fixes issue #63.