code: func UnmarshalFixedJSON(typ reflect.Type, input, out []byte) error {
  	if !isString(input) {
  		return errNonString(typ)
  	}
  	return wrapTypeError(UnmarshalFixedText(typ.String(), input[1:len(input)-1], out), typ)
  }
docs: // UnmarshalFixedJSON decodes the input as a string with 0x prefix. The length of out
  // determines the required input length. This function is commonly used to implement the
  // UnmarshalJSON method for fixed-size types.