Extraction: reduce krml output unless -d/--debug

Karamel extraction prints many warnings when definitions cannot be
extracted, but most of these are benign, since they are just types, or
ghost code, etc. This patch disables these warnings unless some
debugging is enabled.

src/extraction/FStar.Extraction.Krml.fst

@@ -1331,7 +1331,8 @@ let translate_type_decl' env ty: option decl =
           Some (DTypeAbstractStruct name)
         else if assumed then
           let name = string_of_mlpath name in
-          BU.print1_warning "Not extracting type definition %s to KaRaMeL (assumed type)\n" name;
+          if Debug.any () then
+            BU.print1_warning "Not extracting type definition %s to KaRaMeL (assumed type)\n" name;
           // JP: TODO: shall we be smarter here?
           None
         else
@@ -1381,7 +1382,8 @@ let translate_let' env flavor lb: option decl =
       if List.length tvars = 0 then
         Some (DExternal (translate_cc meta, translate_flags meta, name, translate_type env t0, arg_names))
       else begin
-        BU.print1_warning "Not extracting %s to KaRaMeL (polymorphic assumes are not supported)\n" (Syntax.string_of_mlpath name);
+        if Debug.any () then
+          BU.print1_warning "Not extracting %s to KaRaMeL (polymorphic assumes are not supported)\n" (Syntax.string_of_mlpath name);
         None
       end
 
@@ -1405,7 +1407,7 @@ let translate_let' env flavor lb: option decl =
         in
         let name = env.module_name, name in
         let i, eff, t = find_return_type E_PURE (List.length args) t0 in
-        if i > 0 then begin
+        if i > 0 && Debug.any () then begin
           let msg = "function type annotation has less arrows than the \
             number of arguments; please mark the return type abbreviation as \
             inline_for_extraction" in
@@ -1509,7 +1511,8 @@ let translate_decl env d: list decl =
       failwith "todo: translate_decl [MLM_Top]"
 
   | MLM_Exn (m, _) ->
-      BU.print1_warning "Not extracting exception %s to KaRaMeL (exceptions unsupported)\n" m;
+      if Debug.any () then
+        BU.print1_warning "Not extracting exception %s to KaRaMeL (exceptions unsupported)\n" m;
       []
 
 let translate_module uenv (m : mlpath & option (mlsig & mlmodule) & mllib) : file =