Fix Trac #2321: bug in SAT

  This is a fairly substantial rewrite of the Static Argument Transformatoin,
  done by Max Bolingbroke and reviewed and modified by Simon PJ.
  
  * Fix a subtle scoping problem; see Note [Binder type capture]
  * Redo the analysis to use environments
  * Run gentle simlification just before the transformation

compiler/main/DynFlags.hs

@@ -849,7 +849,7 @@ getCoreToDo dflags
     liberate_case = dopt Opt_LiberateCase dflags
     rule_check    = ruleCheck dflags
     vectorisation = dopt Opt_Vectorise dflags
-    -- static_args   = dopt Opt_StaticArgumentTransformation dflags
+    static_args   = dopt Opt_StaticArgumentTransformation dflags
 
     maybe_rule_check phase = runMaybe rule_check (CoreDoRuleCheck phase)
 
@@ -903,8 +903,7 @@ getCoreToDo dflags
     -- may expose extra opportunities to float things outwards. However, to fix
     -- up the output of the transformation we need at do at least one simplify
     -- after this before anything else
-            -- runWhen static_args CoreDoStaticArgs,
-            -- XXX disabled, see #2321
+        runWhen static_args (CoreDoPasses [ simpl_gently, CoreDoStaticArgs ]),
 
         -- initial simplify: mk specialiser happy: minimum effort please
         simpl_gently,