Major refactor to how printer methods work.

Major refactor to how printer methods work.

* Replaced define-unparser-method with define-print-method.  A print method is
  an ordinary procedure that accepts an object and an output port as its
  arguments and prints whatever it wants to.  The translation layer required by
  older unparser methods is no longer needed.

* New standard-print-method is roughly equivalent to simple-unparser-method.

* New bracketed-print-method is roughly equivalent to standard-unparser-method.

* Changed the printer to handle standard-print-method specially when detecting
  cycles and shared structure, so that an object using that print method has its
  parts walked when looking for shared/cyclic structure.