Hide minimal definition for only-method classes

Previously this was not covered by the All xs check since here it is not
actually an All, rather a single Var n.

This also adds the previously missing html-test/src/Minimal.hs.
Fix issue #281

This is a regression from the data family instances change. Data
instances are now distinguished from regular lists by usage of the new
class "inst", and the style has been updated to only apply to those.

I've also updated the appropriate test case to test this a bit better,
including GADT instances with GADT-style records.