html-test/Operators: Clear up ambiguous types
html-test/Operators: Clear up ambiguous types

For reasons that aren't entirely clear a class with ambiguous types was
accepted by GHC <8.0. I've added a functional dependency to clear up
this ambiguity.

Fuuzetsu/haddock,randen/haddock,randen/haddock,idontgetoutmuch/haddock,haskell/haddock,idontgetoutmuch/haddock,haskell/haddock,haskell/haddock,Helkafen/haddock,randen/haddock,niteria/haddock,Fuuzetsu/haddock,Fuuzetsu/haddock,niteria/haddock,Fuuzetsu/haddock,Helkafen/haddock,Helkafen/haddock,idontgetoutmuch/haddock,niteria/haddock,haskell/haddock,haskell/haddock