* Include the operators.h header file in the source files, not in ex.h.

* Include the operators.h header file in the source files, not in ex.h.
  Rationale: It really gets in the way there, when trying to strip down
  the source files for debugging purposes.
* are_ex_trivially_equal(): use this only when reasonable, i.e. when
  an operation on all children was a no-op such that the this pointer is
  returned.