New classes and types for handling asset amounts:
  * Quality opaque type for order book quality
  * Amount replacement for STAmount
  * Amounts, in/out amount pair for offers
  * 'core' namespace with type aliases for Ripple primitives.
New classes for processing offers in order books:
* BookTip provides consume-and-step offer traversal
* OfferStream applies offer business rules and presents offers to callers
* Taker class manages state for the active party during order processing
* Offer class wraps book offers for presentation