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.

Python/future.c

@@ -33,6 +33,8 @@ future_check_features(PyFutureFeatures *ff, node *n, char *filename)
 			ff->ff_nested_scopes = 1;
 		} else if (strcmp(feature, FUTURE_GENERATORS) == 0) {
 			ff->ff_generators = 1;
+		} else if (strcmp(feature, FUTURE_DIVISION) == 0) {
+			ff->ff_division = 1;
 		} else if (strcmp(feature, "braces") == 0) {
 			PyErr_SetString(PyExc_SyntaxError,
 					"not a chance");
@@ -234,6 +236,7 @@ PyNode_Future(node *n, char *filename)
 	ff->ff_last_lineno = -1;
 	ff->ff_nested_scopes = 0;
 	ff->ff_generators = 0;
+	ff->ff_division = 0;
 
 	if (future_parse(ff, n, filename) < 0) {
 		PyMem_Free((void *)ff);