Merge from cc-memout branch. Here are the main points

Merge from cc-memout branch.  Here are the main points

* Add ContextMemoryAllocator<T> allocator type, conforming to
  STL allocator requirements.

* Extend the CDList<> template to take an allocator (defaults
  to std::allocator<T>).

* Add a specialized version of the CDList<> template (in
  src/context/cdlist_context_memory.h) that allocates a list
  in segments, in context memory.

* Add "forward" headers -- cdlist_forward.h, cdmap_forward.h,
  and cdset_forward.h.  Use these in public headers, and other
  places where you don't need the full header (just the
  forward-declaration).  These types justify their own header
  (instead of just forward-declaring yourself), because they
  are complex templated types, with default template parameters,
  specializations, etc.

* theory_engine.h no longer depends on individual theory headers.
  (Instead it forward-declares Theory implementations.)  This is
  especially important now that theory .cpp files depend on
  TheoryEngine (to implement Theory::getValue()).  Previously,
  any modification to any theory header file required *all*
  theories, and the engine, to be completely rebuilt.

* Support memory cleanup for nontrivial CONSTANT kinds.  This
  resolves an issue with arithmetic where memory leaked for
  each distinct Rational or Integer that was wrapped in a Node.