Improve extended slicing support in builtin types and classes. Specifically:

- Specialcase extended slices that amount to a shallow copy the same way as
   is done for simple slices, in the tuple, string and unicode case.

 - Specialcase step-1 extended slices to optimize the common case for all
   involved types.

 - For lists, allow extended slice assignment of differing lengths as long
   as the step is 1. (Previously, 'l[:2:1] = []' failed even though
   'l[:2] = []' and 'l[:2:None] = []' do not.)

 - Implement extended slicing for buffer, array, structseq, mmap and
   UserString.UserString.

 - Implement slice-object support (but not non-step-1 slice assignment) for
   UserString.MutableString.

 - Add tests for all new functionality.

Objects/stringobject.c

@@ -1222,6 +1222,17 @@ string_subscript(PyStringObject* self, PyObject* item)
 		if (slicelength <= 0) {
 			return PyString_FromStringAndSize("", 0);
 		}
+		else if (start == 0 && step == 1 &&
+			 slicelength == PyString_GET_SIZE(self) &&
+			 PyString_CheckExact(self)) {
+			Py_INCREF(self);
+			return (PyObject *)self;
+		}
+		else if (step == 1) {
+			return PyString_FromStringAndSize(
+				PyString_AS_STRING(self) + start,
+				slicelength);
+		}
 		else {
 			source_buf = PyString_AsString((PyObject*)self);
 			result_buf = (char *)PyMem_Malloc(slicelength);