Define OpaqueObject and OpaqueData equality contract

Generating anyxml/anydata properties has a direct implication on DataObject.equals(). Since we currently do not define an equality contract, the default is identity comparison, which is wrong and breaks with proxying – if we generate two proxies, we will end up blowing up if we're not careful.

The contract needs to be defaulted in OpaqueData first: compare the representations and then the values. Implementations of OpaqueValue can then decide how to compare (via a default method).

For OpaqueObjects, we first need to compare implementedInterface() and then compare OpaqueData.