message: objc: fix std::string handling
  arg types may be passed as string instead of std::string
diff: "from_cpp": "[NSString stringWithUTF8String:%(n)s.c_str()]",
  "swift_type": "String"
  },
  + "string": {
  + "cast_to": "std::string",
  + "objc_type": "NSString*",
  + "to_cpp": "std::string(%(n)s.UTF8String)",
  + "from_cpp": "[NSString stringWithUTF8String:%(n)s.c_str()]",
  + "swift_type": "String"
  + },
  "TermCriteria": {
  "objc_type": "TermCriteria*",
  "to_cpp": "%(n)s.nativeRef",
  