Ripple calc

* New uint160 tagged type NodeID.
* Categorize *all* non-generic instances of uint160 as Account, Currency or NodeID.
* Move conversions to and from strings to central .h file.
* Extract macro constants to be functions.
* Get rid of constructors and comparators between tagged uint types.
* New base_uint::copyFrom() allows explicit copying between any uint types
  of the same size.
* And lots of incidental cleaning and removal of unused code.
We were invisibly paying that price even before this megachange - it was simply hidden by the automatic promotion/demotion copies.

I can get rid of this the same way I got rid of the other copies.  However, SerializedTypes has SO many accessors with the word "Field" in the name that I decided for now to stop since it was all together...  pending my better understanding of those different methods!