Do pretty-printing of TyThings via IfaceDecl (Trac #7730)

All the initial work on this was done fy 'archblob' (fcsernik@gmail.com);
thank you!

I reviewed the patch, started some tidying, up and then ended up in a huge
swamp of changes, not all of which I can remember now.  But:

* To suppress kind arguments when we have -fno-print-explicit-kinds,
    - IfaceTyConApp argument types are in a tagged list IfaceTcArgs

* To allow overloaded types to be printed with =>, add IfaceDFunTy to IfaceType.

* When printing data/type family instances for the user, I've made them
  print out an informative RHS, which is a new feature. Thus
        ghci> info T
        data family T a
        data instance T Int = T1 Int Int
        data instance T Bool = T2

* In implementation terms, pprIfaceDecl has just one "context" argument,
  of type IfaceSyn.ShowSub, which says
       - How to print the binders of the decl
         see note [Printing IfaceDecl binders] in IfaceSyn
       - Which sub-comoponents (eg constructors) to print

* Moved FastStringEnv from RnEnv to OccName

It all took a ridiculously long time to do.  But it's done!

1 files changed, 1 insertions, 1 deletions

diff --git a/testsuite/tests/ghci/scripts/T5417.stdout b/testsuite/tests/ghci/scripts/T5417.stdout
index 73d1de932d..1085a1750f 100644
--- a/testsuite/tests/ghci/scripts/T5417.stdout
+++ b/testsuite/tests/ghci/scripts/T5417.stdout
@@ -6,4 +6,4 @@ class C.C1 a where
   type role C.F nominal
   data family C.F a
   	-- Defined at T5417a.hs:5:5
-data C.F (B1 a) -- Defined at T5417.hs:8:10
+data instance C.F (B1 a) = B2 a 	-- Defined at T5417.hs:8:10