implement and document rules for repeated/combined DEFRULE options

* The :when DEFRULE options cannot be repeated (This had already been
  implemented, but the documentation string did not explicitly state
  the fact)

* All DEFRULE options which specify transformations can now be
  repeated and/or combined. The resulting transformation is the
  composition of the supplied transformations in the specified
  order. Only the first transformation can receive bounds via &BOUNDS.

* The system version has been bumped from 0.10 to 0.11 and a feature
  :ESRAP.MULTIPLE-TRANSFORMS is pushed onto *FEATURES* when the system
  is loaded.

* Unit tests have been added: MULTIPLE-TRANSFORMS.{1,2}