1. remove the BoxedTraceback** argument from maybeTracebackHere.  the unwinding.cpp
   code can get it just as easily.
2. combine UNWIND_STATE_OSR and UNWIND_STATE_RERAISE into UNWIND_STATE_SKIPNEXT,
   since they share the same behavior.
3. put the SKIPNEXT behavior in the callers of unwindProcessFrame.  Now unwindProcessFrame
   does nothing but create a PythonFrameIteratorImpl (if it can), and if it can
   calls its callback with it.
4. initialize the unwind state to NORMAL in raiseExc.