Add _PyUnicode_UTF8() and _PyUnicode_UTF8_LENGTH() macros

* Rename existing _PyUnicode_UTF8() macro to PyUnicode_UTF8()
 * Rename existing _PyUnicode_UTF8_LENGTH() macro to PyUnicode_UTF8_LENGTH()
 * PyUnicode_UTF8() and PyUnicode_UTF8_LENGTH() are more strict