Add T_PYSSIZET in structmember.h: This can be used for
Py_ssize_t members.

Simplify the implementation of UnicodeError objects:
start and end attributes are now stored directly as
Py_ssize_t members, which simplifies various get and
set functions.
diff --git a/Include/structmember.h b/Include/structmember.h
index e761f6d..ce5353d 100644
--- a/Include/structmember.h
+++ b/Include/structmember.h
@@ -68,6 +68,7 @@
 #ifdef HAVE_LONG_LONG
 #define T_LONGLONG      17  
 #define T_ULONGLONG      18
+#define T_PYSSIZET       19 /* Py_ssize_t */
 #endif /* HAVE_LONG_LONG */
 
 /* Flags */