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've been playing with auto for a while now (before I got here).

My new theory is that I should *always* use auto when possible - simply because it frees me of the cognitive burden of deciding when I should use it and makes the code as short as possible.

It should take you almost no time to "go looking" for the actual type if your IDE is any good - or even if it isn't, I mainly work in emacs which is less good and it's still not an issue.

But I didn't go looking for trouble here.  I only changed things I was "forced to" - i.e. Accounts and Currencies where I wanted to create code that was good before and after the change of type.