[vsipl++] Cleanup patch

[Jules Bergmann]

Stefan, looks good.  Please commit.  -- Jules

Stefan Seefeld wrote:
> The attached patch does some cleanup in order to enhance
> header independency:
> 
> * view_traits.hpp forward-declares views with all default
>   arguments, which vector.hpp, matrix.hpp, and tensor.hpp
>   then don't issue a second time.
> * dense.hpp doesn't depend on view_traits.hpp
> * expr_functor.hpp depends on expr_binary_operators.hpp
> * matvec.hpp requires promote.hpp and fns_elementwise.hpp
>   to be self-contained.
> 
> Ok to commit ?
> 
> Regards,
>         Stefan
> 
>