Supply branch incomps when building an IfaceClosedSynFamilyTyCon
pprTyThing now has access to incomps. This also causes them to be
written out to .hi files, but that doesn't pose an issue other than a
more faithful bijection between tyThingToIfaceDecl and tcIfaceDecl.
The machinery for displaying axiom incomps was already present but not
in use. Since this is now a thing that pops up in ghci's :info the
format was modified to look like a haskell comment.
Documentation and a test for the new feature included.