Reduce fragility of EnumSet/EnumMap marshalling

The marshallers for EnumSet and EnumMap currently rely on reflection (in specific cases) to obtain the Enum Class from which the set/map was constructed. The current implementation relies on a specific named field.

This is not ideal, as names of fields can vary across JDK implementations and versions.
A less fragile solution involves looking for the field of the expected type, i.e. Class.