Improvement and bug fixes to the new class/c; try again to re-enable it

Fixed:
- class-field-accessor and class-field mutator,
- the interaction between init args contracts and subclasses,
- object=? on wrapped objects (ie if 'this' flows out and
  is compared to the object thatn 'new' returns)
- contract-name for class/c

Also, the code now uses impersonator properties internally to track a
class that is wrapped (instead of a wrapper struct).  This simplifies
a bunch of places in the code and paves the way for classes that have
a class/c contract to be chapeone-of the original class (altho that
doesn't work yet)