This is a basically just a bunch of random cleanups. Despite the name, it doesn't kill FlowObjSpace, but it does hide it from public view.
Highlights:
- replaces FlowObjSpace().build_flow(func) with just build_flow(func)
- cleans up the ArgumentsForTranslation mess in flowspace
- moves ArgumentsForTranslation to rpython.annotator, since it's not used in flowspace any more