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 https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/issues/209
