Replace `ParseArgument<T>` with `Func<ArgumentResult, T>`

[jonsequitur]

    // "ParseArgument" doesn't feel like a delegate name.
    // Guidelines:
    //   * DO add the suffix “EventHandler” to names of delegates that are used in events.
    //   * DO add the suffix “Callback” to names of delegates other than those used as event handlers.
    //   * DO NOT add the suffix “Delegate” to a delegate.
    public delegate T ParseArgument<out T>(ArgumentResult result);