a) Fix black hole bug when doing -dshow-rn-trace
   (Involved reorganising where fixity exports are dealt with
    in RnNames/RnIfaces.)

b) Arrange to apply Lint to imported unfoldings when -dcore-lint

c) Add -fwarn-type-defaults to report use of the defaulting rules for types

d) Make it so that f (error "help) --> error "help", if f is strict
   (Changes in Simplify.lhs.)