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 think it could.  But in this case I actually like the explicit type.  On lines 404 and 453 in for-range loops, and it is so much easier to see what the value_type is that the loops are working on with the type spelled out explicitly on line 400.  I.e. the for loops are using the first and second types of the value_type, which we now know are uint256 and ValidationCounter.  With auto we would have to go looking.