Make gst::Seqnum similar to Gst::ClockTime and represent invalid values in the types
The following discussion from gstreamer/gst-plugins-rs!90 (merged) should be addressed:
I wonder if we should change the type definition of that so that seqnums (and group ids, etc) are never
0 == INVALID, and instead use
Options in the code. Unrelated to your changes here of course, but seems worth doing.
NonZeroU32, and changing the API that can take invalid seqnum (is there any at all?) to an