Implement rule 17

Implement rule 17

* Convert the kerning into a proper right margin for mathnormal, mathscr and
  mathcal (fonts with \fontdimen2 == 0 as per rule 17 from TeXBook).

* Simulate the fact that characters in mathnormal fonts have a 0-width left
  bearing.

* Implement subscript positioning in the case of rule 17 using negative italic
  correction (kerning_).