Add a new :c:type:`PyUnicodeWriter` API to create a Python :class:`str` object:

* :c:func:`PyUnicodeWriter_Create`.
* :c:func:`PyUnicodeWriter_Discard`.
* :c:func:`PyUnicodeWriter_Finish`.
* :c:func:`PyUnicodeWriter_WriteChar`.
* :c:func:`PyUnicodeWriter_WriteUTF8`.
* :c:func:`PyUnicodeWriter_WriteUCS4`.
* :c:func:`PyUnicodeWriter_WriteWideChar`.
* :c:func:`PyUnicodeWriter_WriteStr`.
* :c:func:`PyUnicodeWriter_WriteRepr`.
* :c:func:`PyUnicodeWriter_WriteSubstring`.
* :c:func:`PyUnicodeWriter_Format`.
* :c:func:`PyUnicodeWriter_DecodeUTF8Stateful`.

Patch by Victor Stinner.
