Issue #15609: Optimize str%args for integer argument

- Use _PyLong_FormatWriter() instead of formatlong() when possible, to avoid
   a temporary buffer
 - Enable the fast path when width is smaller or equals to the length,
   and when the precision is bigger or equals to the length
 - Add unit tests!
 - formatlong() uses PyUnicode_Resize() instead of _PyUnicode_FromASCII()
   to resize the output string