think of ways to limit use of type synonyms.

[fisx]

as @np has just noted while working on liqd/thentos, configifier does not offer a good way to write class instances for config types. turning on -XTypeSynonymInstances is a good solution, but having an ADT instead of the Tagged a type aliases would be even nicer.

this should probably be considered in the context of a larger re-design as sketched in #15.