Restore old function names for compatibility; fix #5070

Rename new functions:

- `mrb_convert_type(mrb,val,type,tname,method)`
   => `mrb_type_convert(mrb,val,type,tname,method)`
- `mrb_check_convert_type(mrb,val,type,tname,method)`
   => `mrb_type_convert_check(mrb,val,type,tname,method)`

Old names are defined by macros (support `tname` drop and
`char*` => `mrb_sym` conversion).

src/string.c

@@ -1100,7 +1100,7 @@ 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, MRB_SYM(to_s));
+    return mrb_type_convert(mrb, str, MRB_TT_STRING, MRB_SYM(to_s));
   }
 }