bpo-27129: Use instruction offsets, not byte offsets, in bytecode and internally. (GH-25069)

* Use instruction offset, rather than bytecode offset. Streamlines interpreter dispatch a bit, and removes most EXTENDED_ARGs for jumps.

* Change some uses of PyCode_Addr2Line to PyFrame_GetLineNumber

Tools/gdb/libpython.py

@@ -947,7 +947,7 @@ class PyFrameObjectPtr(PyObjectPtr):
             return self.f_lineno
 
         try:
-            return self.co.addr2line(self.f_lasti)
+            return self.co.addr2line(self.f_lasti*2)
         except Exception:
             # bpo-34989: addr2line() is a complex function, it can fail in many
             # ways. For example, it fails with a TypeError on "FakeRepr" if