This ticket is about making Serializers and Unserializers aware of the possibility of normalized values in the JSON rendering of Snaks.

PropertyValueSnaks currently look like this:

"mainsnak":{"snaktype":"value","property":"P646","datatype":"string",
    "datavalue":{"value":"/m/0g5n7","type":"string"}
}

The normalized snak value would be added using an additional key on the same level as "datavalue", called "datavalue-normalized" (or some such):

"mainsnak":{"snaktype":"value","property":"P646","datatype":"string",
    "datavalue":{"value":"/m/0g5n7","type":"string"},
    "datavalue-normalized":{"value":"https://www.freebase.com/m/0g5n7","type":"string"},
}

Rough spec after discussion with Thiemo and Jan, 2015-06-17:

• ExtendedPropertyValueSnak extends PropertyValueSnak ("Extended" is silly, find a better name). ExtendedPropertyValueSnak->values: map of "roles" (original, normalized, uri, url, iso, etc) to DataValue objects. T102791
• Serializer/Unserializer get support for ExtendedPropertyValueSnak. T102833
• Support for derived values (ExtendedPropertyValueSnak, FallbackTerm) can be disabled in serializer/unserializer (should be disabled for API input, and db storage). T102789, T102788
• ValueExtender service, with specialized extenders for specific DataType. Used to manipulate Statements before serialization. T102835