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
This allows to use MappedBuffer and similar types to be properly
debug-printed.
Also change VideoFrame/VideoFrameRef/RTPBuffer from a tuple struct to a
struct with proper field names to make the code easier to understand.
They can actually be shared with multiple threads at the same time
safely as all functions requiring an immutable reference are
thread-safe.
OTOH TypeFind can't be shared safely between different threads as not
all implementations of the TypeFind struct are thread-safe.
