Note [Implicit axioms]

See also Note [Implicit TyThings] in GHC.Types.TyThing
* A CoAxiom arising from data/type family instances is not "implicit".
  That is, it has its own IfaceAxiom declaration in an interface file

* The CoAxiom arising from a newtype declaration *is* "implicit".
  That is, it does not have its own IfaceAxiom declaration in an
  interface file; instead the CoAxiom is generated by type-checking
  the newtype declaration