Removed the Enum instance for Dimensional.

It seems like the `Enum` instance needlessly exposes the implementation details of `Dimensional`, because there is no physical reason why `succ (1 *~ meter)` should be 2 meters, while `succ (1 *~ foot)` is 1.3048 meters.

Perhaps some analog of `enumFromTo` that increments by a specified quantity might better suit any existing clients? Or `[0..1000] *~~ meter`?