* Renamed _fd_* output functions to fd__* (meaning that they are
  intended to be private to the library) and put them in the new
  util.h file.

* Use fd__trace() and -DTRACE to produce tracing messages. fd__debug()
  is now reserved for real debugging output.

* Use fd_const() to create variables with singleton domains.

* New poly-le-k constraint.

* Include paccs.h where possible.

* Committed to INDEX_IN_POOL and CONSTRAINT_CLASS.

* Improved domain filtering for le, lt, poly-eq, poly-eq-k and
  var-eq-minus.

* Decoupled recording that a constraint is entailed from using
  CONSTRAINT_TEMPS. (Use -DDISABLE_ENTAILED to disable.)