support more coercions

[zliu41]

The plugin supports only a handful of coercions in categorizeFun. One in particular that crops up is AxiomInstCo.

So far, AxiomInstCo has always been the result of a type class that has a single method and no super-classes. There is a workaround for this case.

1. make the type class "more complex" by either adding a dummy method or adding a super-class, like class EmptyClass a => SimpleClass a where ...
2. include a comment referencing this ticket.

(Extricated from https://kitty-hawk.atlassian.net/browse/SW-3013)