Add basic support to populate pre-existing C# objects when deserializing

[krisrok]

Add IDeserializer.PopulateObject() APIs Add currentValue parameter to INodeDeserializers and IValueDeserializers Add checks to only create a new instance if currentValue is empty Add ways to allow the user to configure population of pre-existing collections: DeserializerBuilder.WithCollectionPopulationOptions()

Note: Pre-existing collection items do not get populated. Pre-existing collections can be re-used though, see DeserializerBuilder.WithCollectionPopulationOptions()

Todo: Allow to populate pre-existing collection items. This needs some refactoring of the way the Array-, Collection- and DictionaryNodeDeserializer work.

#130