Add XFL Tools

[zgrguric]

Add method to construct or deconstruct XLS-17d (XFL) floating point values.

See https://github.com/RichardAH/xfl-tools

[AlexanderBuzz]

@zgrguric Is this still interesting?

[zgrguric]

I've have done own implementation, its not required to be in XRPL_PHP for now, we can close this and see if there is any interest in future.