gstreamer: Use more accurate types for Seqnum, GroupId and MetaSeqnum
For the latter introduce an actual opaque type that allows using them for comparison purposes but is not just a plain u64.
For the former represent them as opaque type around an NonZeroU32. 0 is the invalid case and does not happen in the majority of functions. Where it can happen, represent this case by using an Option<_> instead.
This makes it harder to mis-use these types.
Fixes #209 (closed)
CC @meh