boost::compute::valarray should support std::begin and std::end

[jeffhammond]

I want to be able to copy a compute::valarray to a std::valarray like this:

std::valarray<float> h(0.0f,1000);
compute::valarray<float> d(1.0f,1000);
compute::copy(std::begin(d), std::end(d), std::begin(h));

This fails because begin() and end() are private (http://www.boost.org/doc/libs/1_62_0/libs/compute/doc/html/boost/compute/valarray.html).

In contrast, begin() and end() are supported for std::valarray since C++11 (http://en.cppreference.com/w/cpp/numeric/valarray).

I solved the issue locally with the following trivial change.

//private:
    buffer_iterator<T> begin() const
    {
        return buffer_iterator<T>(m_buffer, 0);
    }

    buffer_iterator<T> end() const
    {
        return buffer_iterator<T>(m_buffer, size());
    }

Is this an acceptable change? It would be nice if boost::compute::valarray behaved like C++11 std::valarray.

I can contribute the trivial change if that is sufficient. If a more complicated one is required, I'll do my best.

[jszuppe]

Yeah, I think you can make them public if BOOST_COMPUTE_USE_CPP11 is defined. You can also look at https://github.com/boostorg/compute/issues/707 if you are interested in boost::compute::valarray.