Rework `ViewDataDictionary` to ensure `ModelMetadata` is never `null` (#1466)

This is the reanimation of #1453, now done against dev.  No significant changes from that PR after I addressed comments there.

See individual commits for details.

/cc @pranavkm @rynowak @yishaigalatzer 
Couldn't we have derived types call into `ViewDataDictionary([NotNull] ViewDataDictionary source, object model, Type declaredModelType)`? It saves us the number of ctors to maintain and the corresponding documentation overhead.