* 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.
* Rename unity files
* Move some modules to new subdirectories
* Remove obsolete Visual Studio project files
* Remove obsolete coding style and TODO list