Generalize operator.indexOf (PySequence_Index) to work with any

iterable object.  I'm not sure how that got overlooked before!

Got rid of the internal _PySequence_IterContains, introduced a new
internal _PySequence_IterSearch, and rewrote all the iteration-based
"count of", "index of", and "is the object in it or not?" routines to
just call the new function.  I suppose it's slower this way, but the
code duplication was getting depressing.

Objects/typeobject.c

@@ -2559,7 +2559,8 @@ slot_sq_contains(PyObject *self, PyObject *value)
 	}
 	else {
 		PyErr_Clear();
-		return _PySequence_IterContains(self, value);
+		return _PySequence_IterSearch(self, value,
+					      PY_ITERSEARCH_CONTAINS);
 	}
 }