* Split STAmount out of SerializedTypes.h
* New concept of "Issue consistency": when either both or neither of its
currency and account are XRP.
* Stop checking for consistency of Issue in its constructor.
* Clarification of mIsNative logic in STAmount.
* Usual cleanups.
* Better automatic conversions to and from tagged uint160 varints.
* Start using tagged variants of uint160 for Currency, Account.
* Comments from 2014/6/11 RippleCalc session.