Improved __future__ parser; still more to do

Makefile.pre.in: add target future.o

Include/compile.h: define PyFutureFeaters and PyNode_Future()
                   add c_future slot to struct compiling

Include/symtable.h: add st_future slot to struct symtable

Python/future.c: implementation of PyNode_Future()

Python/compile.c: use PyNode_Future() for nested_scopes support

Python/symtable.c: include compile.h to pick up PyFutureFeatures decl

Include/symtable.h

@@ -30,6 +30,7 @@ struct symtable {
 	int st_errors;           /* number of errors */
 	char *st_private;        /* name of current class or NULL */
 	int st_tmpname;          /* temporary name counter */
+	PyFutureFeatures *st_future; /* module's future features */
 };
 
 typedef struct _symtable_entry {