moved stan/diff/fvar.hpp to stan/diff/fwd.hpp, stan/diff/var.hpp to stan/diff/var.hpp, stan/diff/matrix.hpp to stan/diff/rev/matrix.hpp
moved stan/diff/fvar.hpp to stan/diff/fwd.hpp, stan/diff/var.hpp to stan/diff/var.hpp, stan/diff/matrix.hpp to stan/diff/rev/matrix.hpp