mruby.h: obsolete mrb_to_str().

Replace them by `mrb_ensure_string_type()`.

src/error.c

@@ -27,7 +27,7 @@ mrb_exc_new(mrb_state *mrb, struct RClass *c, const char *ptr, size_t len)
 MRB_API mrb_value
 mrb_exc_new_str(mrb_state *mrb, struct RClass* c, mrb_value str)
 {
-  mrb_to_str(mrb, str);
+  mrb_ensure_string_type(mrb, str);
   return mrb_obj_new(mrb, c, 1, &str);
 }