WIP - when accessing specific property, only deserialize that one

[mpollmeier]

• before this change, the entire element would be deserialized
• because the majority of properties are actually unused, this should save us a lot of time
• best used in combination with cpg2overflowdb, i.e. we don't first initialize the entire graph, but only the element refs
• change for binary format: instead of writing string property keys we index the properties, using Nil values to fill gaps
• since we can hard code the property index and type in the specific classes, there's no need to have this in the binary format
• internally I'm intending to use only the 'readSpecificProperty' rather than reading all properties - that'll come back as a traversal optimization later though.