A generalisation of EnumMap to dependent key and value types. The key type
needs to be an instance of class Enum1, a generalisation of the Enum class.
Most of the API of EnumMap/IntMap is supported, usually by wrapping IntMap
operations in coercions accompanied by some runtime type-consistency
assertions.
Simpler but more restrictive Enum1 class definition (one can always set type Enum1Info f = f, however, perhaps with a performance cost compared to the old definition).
