Use unicode arrows with -fprint-unicode-syntax

See #8959 (closed), this is one more place where we can pretty-print Unicode syntax.

Test Plan: validate

Reviewers: nomeata, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, carter

GHC Trac Issues: #8959 (closed)

Differential Revision: https://phabricator.haskell.org/D5439

compiler/hsSyn/HsTypes.hs

@@ -1444,7 +1444,7 @@ ppr_fun_ty ty1 ty2
   = let p1 = ppr_mono_lty ty1
         p2 = ppr_mono_lty ty2
     in
-    sep [p1, text "->" <+> p2]
+    sep [p1, arrow <+> p2]
 
 --------------------------
 ppr_tylit :: HsTyLit -> SDoc