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/token.h

@@ -55,10 +55,12 @@ extern "C" {
 #define LEFTSHIFTEQUAL	45
 #define RIGHTSHIFTEQUAL	46
 #define DOUBLESTAREQUAL	47
+#define DOUBLESLASH	48
+#define DOUBLESLASHEQUAL 49
 /* Don't forget to update the table _PyParser_TokenNames in tokenizer.c! */
-#define OP		48
-#define ERRORTOKEN	49
-#define N_TOKENS	50
+#define OP		50
+#define ERRORTOKEN	51
+#define N_TOKENS	52
 
 /* Special definitions for cooperation with parser */