Add functions PyUnicode_Append() and PyUnicode_AppendAndDel() that mirror

PyString_Concat() and PyString_ConcatAndDel() (the name PyUnicode_Concat()
was already taken).

Change PyObject_Repr() to always return a unicode object.

Update all repr implementations to return unicode objects.

Add a function PyObject_ReprStr8() that calls PyObject_Repr() and converts
the result to an 8bit string.

Use PyObject_ReprStr8() where using PyObject_Repr() can't be done
straightforward.

Objects/codeobject.c

@@ -300,7 +300,7 @@ code_repr(PyCodeObject *co)
 	PyOS_snprintf(buf, sizeof(buf),
 		      "<code object %.100s at %p, file \"%.300s\", line %d>",
 		      name, co, filename, lineno);
-	return PyString_FromString(buf);
+	return PyUnicode_FromString(buf);
 }
 
 static PyObject *