FeatureCollection and Feature should be top-level object classes about geojson

[jmikola]

The namespaces are just there to separate classes into logical packages and aren't meant to enforce a class hierarchy (although it is often consistent with one). I could have put both collection classes in a GeoJson\Collection namespace, but I thought it was more intuitive to have all of the geometry or feature-related classes in one place.

The Feature and FeatureCollection classes do not extend a common Feature class

The collection classes shouldn't extend their non-collection counterparts, as they are only GeoJson objects (outlined in the spec). I'm not sure if your comment was motivated by the GeometryCollection's class hierarchy, but the fact that it extends Geometry instead of GeoJson is actually a bug. I'm tracking that in #13.