Implement PEP 238 in its (almost) full glory.

This introduces:

- A new operator // that means floor division (the kind of division
  where 1/2 is 0).

- The "future division" statement ("from __future__ import division)
  which changes the meaning of the / operator to implement "true
  division" (where 1/2 is 0.5).

- New overloadable operators __truediv__ and __floordiv__.

- New slots in the PyNumberMethods struct for true and floor division,
  new abstract APIs for them, new opcodes, and so on.

I emphasize that without the future division statement, the semantics
of / will remain unchanged until Python 3.0.

Not yet implemented are warnings (default off) when / is used with int
or long arguments.

This has been on display since 7/31 as SF patch #443474.

Flames to /dev/null.

Include/object.h

@@ -161,6 +161,12 @@ typedef struct {
 	binaryfunc nb_inplace_and;
 	binaryfunc nb_inplace_xor;
 	binaryfunc nb_inplace_or;
+
+	/* The following require the Py_TPFLAGS_HAVE_CLASS flag */
+	binaryfunc nb_floor_divide;
+	binaryfunc nb_true_divide;
+	binaryfunc nb_inplace_floor_divide;
+	binaryfunc nb_inplace_true_divide;
 } PyNumberMethods;
 
 typedef struct {
@@ -396,7 +402,7 @@ given type object has a specified feature.
 /* tp_iter is defined */
 #define Py_TPFLAGS_HAVE_ITER (1L<<7)
 
-/* Experimental stuff for healing the type/class split */
+/* New members introduced by Python 2.2 exist */
 #define Py_TPFLAGS_HAVE_CLASS (1L<<8)
 
 /* Set if the type object is dynamically allocated */