handle all Enums as immutable types

from Jörg on user thread:

"immutability is normally something that is a very special case tied to special classes. It's just that Enums represent a complete hierarchy of class types that should be treated in this way. Therefore I am more inclined to add something like this to the mappers directly."

in general it woudl be nice to handle all enum types as immutable classes so that references are not used.