Refactor XRPAmount and MPTAmount to use a common base class. Consider renaming to XRPNumber/MPTNumber/IOUNumber.

[gregtatcam]

Summary

They should be called MPTNumber and XRPNumber because they represent quantities, while STAmount represents a quantity plus an asset / issue / unit (however you want to think of it). These are more like Number, and convert directly to and from it.
All of the arithmetic is moving to Number after the switchover anyway. In due time, when we retire that amendment, effectively locking it in permanently, then I think we can remove MPTAmount and XRPAmount.