Creates a new instance from the given string.
If the string passed as `stringValue` does not correspond to any instance
of this type, the result is `nil`.
- parameter stringValue: The string value of the desired key.
Creates a new instance from the specified integer.
If the value passed as `intValue` does not correspond to any instance of
this type, the result is `nil`.
- parameter intValue: The integer value of the desired key.