Use new specifiers/modifiers of `mrb_vfromat()`

The binary sizes (gems are only `mruby-bin-mruby`) are reduced slightly in
my environment than before the introduction of new specifiers/modifiers
(5116789a) with this change.

  ------------+-------------------+-------------------+--------
   BINARY     | BEFORE (5116789a) |   AFTER (This PR) |  RATIO
  ------------+-------------------+-------------------+--------
   mruby      |      593416 bytes |      593208 bytes | -0.04%
   libmruby.a |      769048 bytes |      767264 bytes | -0.23%
  ------------+-------------------+-------------------+--------

BTW, I accidentally changed `tasks/toolchains/visualcpp.rake` at #4613,
so I put it back.

1 files changed, 2 insertions, 3 deletions

diff --git a/src/variable.c b/src/variable.c
index e6f2f397e..32416da4e 100644
--- a/src/variable.c
+++ b/src/variable.c
@@ -446,7 +446,7 @@ MRB_API void
 mrb_iv_name_sym_check(mrb_state *mrb, mrb_sym iv_name)
 {
   if (!mrb_iv_name_sym_p(mrb, iv_name)) {
-    mrb_name_error(mrb, iv_name, "'%S' is not allowed as an instance variable name", mrb_sym2str(mrb, iv_name));
+    mrb_name_error(mrb, iv_name, "'%n' is not allowed as an instance variable name", iv_name);
   }
 }
 
@@ -654,8 +654,7 @@ mrb_mod_cv_get(mrb_state *mrb, struct RClass *c, mrb_sym sym)
       if (given) return v;
     }
   }
-  mrb_name_error(mrb, sym, "uninitialized class variable %S in %S",
-                 mrb_sym2str(mrb, sym), mrb_obj_value(cls));
+  mrb_name_error(mrb, sym, "uninitialized class variable %n in %C", sym, cls);
   /* not reached */
   return mrb_nil_value();
 }