openingsInView function
host's openings in view: its referrers carrying OpeningParams whose
host is host, ascending by handle, with their parameters.
Implementation
List<(Handle, OpeningParams)> openingsInView(
ParametricView view, Handle host) =>
[
for (final h in view.referrers(host))
if (view.paramsOf<OpeningParams>(h) case final o? when o.host == host)
(h, o),
];