* Simplify the code: replace 4 steps with one unique step using the
   _PyUnicodeWriter API. PyUnicode_Format() has the same design. It avoids to
   store intermediate results which require to allocate an array of pointers on
   the heap.
 * Use the _PyUnicodeWriter API for speed (and its convinient API):
   overallocate the buffer to reduce the number of "realloc()"
 * Implement "width" and "precision" in Python, don't rely on sprintf(). It
   avoids to need of a temporary buffer allocated on the heap: only use a small
   buffer allocated in the stack.
 * Add _PyUnicodeWriter_WriteCstr() function
 * Split PyUnicode_FromFormatV() into two functions: add
   unicode_fromformat_arg().
 * Inline parse_format_flags(): the format of an argument is now only parsed
   once, it's no more needed to have a subfunction.
 * Optimize PyUnicode_FromFormatV() for characters between two "%" arguments:
   search the next "%" and copy the substring in one chunk, instead of copying
   character per character.