#1629: Renamed Py_Size, Py_Type and Py_Refcnt to Py_SIZE, Py_TYPE and Py_REFCNT. Macros for b/w compatibility are available.

backwards compatibility. Add Py_Refcnt, Py_Type, Py_Size, and
PyVarObject_HEAD_INIT.

exposed in header files.  Fixed a few comments in these headers.

As we might have expected, writing down invariants systematically exposed a
(minor) bug.  In this case, function objects have a writeable func_code
attribute, which could be set to code objects with the wrong number of
free variables.  Calling the resulting function segfaulted the interpreter.
Added a corresponding test.

Thanks to Skip Montanaro and Kalle Svensson for the patches.

Noted by Jason Orendorff, SF #520768.

Bug fix candidate for 2.1 & 2.2.

A cell contains a reference to a single PyObject.  It could be
implemented as a mutable, one-element sequence, but the separate type
has less overhead.