reduce cost of unsigned type conversion

src/Convert.hs

@@ -74,6 +74,7 @@ finalPhases _ =
     , Convert.EmptyArgs.convert
     , Convert.FuncRet.convert
     , Convert.TFBlock.convert
+    , Convert.Unsigned.convert
     , Convert.StringType.convert
     ]
 
@@ -96,7 +97,6 @@ mainPhases tops selectExclude =
     , Convert.Struct.convert
     , Convert.Typedef.convert
     , Convert.UnpackedArray.convert
-    , Convert.Unsigned.convert
     , Convert.Wildcard.convert
     , Convert.Enum.convert
     , Convert.StringParam.convert

src/Convert/Unsigned.hs

@@ -18,7 +18,8 @@ convert =
     map $
     traverseDescriptions $
     traverseModuleItems $
-    traverseTypes $ traverseNestedTypes convertType
+    -- doesn't need to visit nested types, as they have been elaborated
+    traverseTypes convertType
 
 convertType :: Type -> Type
 convertType (Implicit        Unsigned rs) = Implicit        Unspecified rs