Refcounted headers

The idea here is to export full sequence and frame headers along with Dav1dPicture to API callers. We should probably document relevant members of Dav1dSequenceHeader and Dav1dFrameHeader or output simplified and well-documented sub-structures to make use in certain types of applications easier (see e.g. !246 (merged)). This is more a RfP than a formal MR, so feel free to give substantial feedback. We might even decide this is a terrible idea, in which case we'll simply not merge this.