Add new method in `Deserializers.Base` to support `ReferenceType`

[cowtowncoder]

(and ditto for Serializers.Base)

Since 2.6 added new concept of referential types; and since referential types are really quite a distinct value type, it makes sense to improve registration of serializers, deserializers, for the same.
But since Serializers and Deserializers are interfaces, it is not easy to add new methods without breaking backwards compatibility. But there are Base classes, which could relatively easily be augmented to have new methods called for instances of ReferenceType. This would also require addition of another "bean" access method.

It is not 100% clear this is the best approach to take; if not, can close the issue. But I want to record this to keep the idea around until it can be evaluated.

[cowtowncoder]

Oddly enough I must have added this in 2.7 for some reason. Oh well.