* Add some protected ContextObj accessors for ContextObj-derived classes:

* Add some protected ContextObj accessors for ContextObj-derived classes:
  + Context* getContext() -- gets the context
  + ContextMemoryManager* getCMM() -- gets the CMM
  + int getLevel() -- the scope level of the ContextObj's most recent update
  + bool isCurrent() -- true iff the most recent update is the current top level

  In particular, the ContextObj::getCMM() call cleans up by TheoryUF's
  ECData::addPredecessor() function substantially (re: code review bug #64).

* Fix serious bugs in context operations that corrupted the ContextObj
  linked lists.  Closes bug #85.

* Identified a bug in the way objects of the "Link" class are
  allocated; see bug #96.

* Re-enable context white-box tests that ensure proper links in linked
  lists.  Closes bug #86.

* Re-enable CDMap<>::emptyTrash().  Closes bug #87.

* Add a tracing option (-t foo or --trace foo) to the driver to enable
  Trace("foo") output stream.  -d foo implies -t foo.

* Minor clean-up of some TheoryUF code; addition of some documentation
  (re: code review bug #64).

* Address some things that caused Doxygen discomfort.

* Address an issue raised in NodeManager's code review (bug #65).

* Remove an inaccurate comment in Attribute code (re: code review bug #61).