This makes some API changes to AutoDiffBlock.
- Add overload for the constant() constructor taking rvalue ref.
- Add overload for the variable() constructor taking rvalue ref.
- Make the function() constructor *require* rvalue refs.
- Add a swap() function.
The remaining changes in this commit are follow-ups especially
to the third change (adding std::move in many places), and
some removal of unnecessary block pattern arguments from calls to
the constant() static method.
Specifically,
- #include <config.h> where appropriate (all .cpp files)
- Adjust include statements to account for sub-directory locations
of .hpp files.
