refactor: replace char_u with char

Work on https://github.com/neovim/neovim/issues/459

src/nvim/api/vimscript.c

@@ -304,7 +304,7 @@ Object nvim_call_dict_function(Object dict, String fn, Array args, Error *err)
     }
     fn = (String) {
       .data = di->di_tv.vval.v_string,
-      .size = STRLEN(di->di_tv.vval.v_string),
+      .size = strlen(di->di_tv.vval.v_string),
     };
   }