Add `JacksonException.Reference` (demote from `JsonMappingException`/`DatabindException`)

[cowtowncoder]

In order to add better (optional) logical location information for all Jackson, it is necessary to move Reference helper type down from jackson-databind into jackson-core. While it could be added as a main-level class, it probably makes more sense as a static inner class for JacksonException, the new uber-base class for exceptions.

It will then also make sense to move related functionality down the inheritance chain.