Change the arguments of following implicit conversion functions:

- `mrb_convert_type`
- `mrb_check_convert_type`

Those function no longer take `tname` string representation of desired
type, and take method symbols instead of `const char*` names. This is
incompatible change. I hope no third-party gems use those functions.

src/string.c

@@ -1126,7 +1126,7 @@ mrb_str_to_str(mrb_state *mrb, mrb_value str)
   case MRB_TT_MODULE:
     return mrb_mod_to_s(mrb, str);
   default:
-    return mrb_convert_type(mrb, str, MRB_TT_STRING, "String", "to_s");
+    return mrb_convert_type(mrb, str, MRB_TT_STRING, MRB_SYM(to_s));
   }
 }