only include CythonFunction utility code if it's really used, might be unused if .pxd overrides all def functions to cdef

automatically replace calls to builtin.__contains__() with the corresponding C-API function and enable optimisation of unbound builtin methods for subtypes

automatically convert dict.iter*() and dict.view*() methods to .keys/values/items() in Py3 for typed dicts

Add unicode slicing support

Add none check for unicode slicing, remove duplicate code, add test for non-ascii unicode strings and None