Issue Description:

Currently, PropertyListDecoder can only decode from instances of Data.
Internally, it calls PropertyListSerialization.propertyList(from: data, options: [], format: &format) on that data to get a top level object, then calls an internal function decode<T>(_ type: T.Type, fromTopLevel container: Any) to perform the actual decoding.

I think it would make sense to make that function public, as sometimes you already have a property list object that you want to decode into the correct swift classes. The only way to do that right now is to wastefully re-serialize it to Data first.