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/opcode.h

@@ -29,6 +29,10 @@ extern "C" {
 #define BINARY_ADD	23
 #define BINARY_SUBTRACT	24
 #define BINARY_SUBSCR	25
+#define BINARY_FLOOR_DIVIDE 26
+#define BINARY_TRUE_DIVIDE 27
+#define INPLACE_FLOOR_DIVIDE 28
+#define INPLACE_TRUE_DIVIDE 29
 
 #define SLICE		30
 /* Also uses 31-33 */