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.

Include/pyerrors.h

@@ -34,8 +34,8 @@ typedef struct {
     PyObject *message;
     PyObject *encoding;
     PyObject *object;
-    PyObject *start;
-    PyObject *end;
+    Py_ssize_t start;
+    Py_ssize_t end;
     PyObject *reason;
 } PyUnicodeErrorObject;
 #endif