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

Python/formatter_unicode.c

@@ -757,7 +757,8 @@ format_string_internal(PyObject *value, const InternalFormatSpec *format,
         goto done;
     }
 
-    if (format->width == -1 && format->precision == -1) {
+    if ((format->width == -1 || format->width <= len)
+        && (format->precision == -1 || format->precision >= len)) {
         /* Fast path */
         return _PyUnicodeWriter_WriteStr(writer, value);
     }