Autobridging:
Complete implementation of bridged offers crossings. While processing an offer
A:B we consider both the A:B order book and the combined A:XRP and XRP:B books
and pick the better offers. The net result is better liquidity and potentially
better rates.

* Rearchitect core::Taker to perform direct and bridged crossings.
* Compute bridged qualities.
* Implement a new Bridged OfferCreate transactor.
* Factor out common code from the Bridged and Direct OfferCreate transactors.
* Perform flow calculations without losing accuracy.
* Rename all transactors.
* Cleanups.