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.
I did make this change just to be nice, but I have to say that I'm starting to wonder if consts on locals really are a good idea.  They're more stuff to read, and they just don't seem to convey any interesting information...  we read so much code as it is and I'm wanting to only see "the good bits".

But I made the change anyway... :-)