Factor common object serialization and deserialization logic out of IObject implementations

[exarkun]

All Kubernetes objects have a kind and apiVersion. Also, these values are fixed per-type (since they basically tell you what the type is). These fields (currently) need special handling in from_raw and to_raw and the handling is essentially the same across all types.

So, instead of making every type duplicate this logic, introduce some APIs that operate on IObject providers and apply this logic.

Something along the lines of object_from_raw(object_to_raw(any_iobject)) == any_iobject.