- Define-slot can now be nested inside fill-slot, so that a macro can
  call another macro and use one of its slots to fill that other
  macro's slots.  (This is essential for sensible semantics of nested
  macros.)

- Inside a use-macro, if the macro defines a slot that's not filled,
  the define-slot turns into fill-slot.  (This is just a convenience
  feature.)

To implement this, the current macro call and its slots are turned
into a stack (they already had stack behavior but the stack was
inaccessible) and added a variable indicating the macro currently
being defined (if any).

In the process, refactored startTagCommon a bit, turning the decision
procedure for argument replacement into a separate method
(attrAction()).

Added a safeguard to the attribute-wrapping feature: if the alignment
column is too far to the right (half of the wrap column), change
alignment to 4 spaces.  This solves the problem reported by Gregor
Hoffleit with attribute wrapping in a long line containing several
elements.