An alternative to the unity build, the classic build compiles each
translation unit individually. This adds more modules to the classic build:
* Remove unity header app.h
* Add missing includes as needed
* Remove obsolete NodeStore backend code
* Add app/, core/, crypto/, json/, net/, overlay/, peerfinder/ to classic build
* Replace SYSTEM_NAME and other macros with C++ constructs
* Remove RIPPLE_ARRAYSIZE and use std::extent or ranged for loops
* Remove old-style, unused offer crossing unit test
* Make STAmount::saFromRate free and remove default argument
* Restrict to 80-columns and other style cleanups.
* Make pathfinding a free function and hide the class Pathfinder.
* Split off unrelated utility functions into separate files.
Conflicts:
src/ripple/rpc/handlers/RipplePathFind.cpp