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.
AFAIK "ACCOUT_ONE" (or now noAccount()) is used as a indication that there is not actual account necessary at this position, kinda like a "Null" value (https://ripple.com/wiki/Accounts#ACCOUNT_ONE). I'm proposing to leave it out entirely instead of reporting a base58 + checksum encoded 1 (rrrrrrrrrrrrrrrrrrrrBZbvji) as a placeholder. Alternatively report something not-base58 encoded to make it clear this is not something that actually is expected to exist on the ledger (which makes this especially confusing since ACCOUT_ONE actually has been funded with XRP: https://ripple.com/graph/#rrrrrrrrrrrrrrrrrrrrBZbvji)