Make TWO-SUM a macro, just like we did for QUICK-TWO-SUM.

All RT tests pass on CMUCL and Allegro.

qd-package.lisp:
o Don't import C::TWO-SUM anymore.

qd-dd.lisp:
o Make TWO-SUM a macro.

qd.lisp
o Add TWO-SUM macro for CMUCL (which just calls C::TWO-SUM).
o Update all uses of TWO-SUM to use the macro appropriately.