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 don't like `ACCOUNT_ONE` but I don't know that I like `noAccount` either. The name is somewhat accurate, I guess, but it just comes across weird. 

Can we please consider some alternatives?