RefChanger

RefChanger

RefChanger temporarily assigns a value to a non-const reference of any
kind. RefChanger provides a flexible and uniform generalisation of the various
scope guards previously derived from the old Changer class in MetricsInfo.h.

As before, a temporary assignment lasts as long as the Changer object lives. But
the new Changer is movable. In particular, contorsions are no longer needed to
change a private field. Special code can be moved into the appropriate classes,
and it is no longer necessary to create a new class for each specific use.

Syntax change:

   FontSetChanger dummy(mi.base, value);
-> Changer dummy = mi.base.changeFontSet(value);

New function for generating arbitrary Changers:

  Changer dummy = make_change(ref, val, condition);

Bugfix:

* Fix the display of \displaystyle{\substack{\frac{xyz}{}}} (missing style
  change).