fix: improve set_result() proposal:

fix: improve set_result() proposal:

- make it async on async cursors (consistently with `results()`,
  inconsistently with `nextset()`, but the latter being sync on async
  cursors was a design mistake);
- raise IndexError, consistently with sequences, better error message;
- fix test, which were simply broken and the OP didn't bother to fix
  them;
- documentation improved.