Get rid of uint160 in favor of Currency, Account and NodeID.

* New uint160 tagged type NodeID.
* Categorize *all* non-generic instances of uint160.
* 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.