Add buffer(Container) constructor

[nilsfriess]

This PR adds a missing constructor to the buffer class (see the description in the spec here). This constructor allows to construct buffers from containers for which std::data(container) and std::size(container) are valid, e.g., std::vector or std::array.

As described by the spec, write_back is disabled if std::data(container) does return a const pointer, and enabled otherwise. Because of this extra handling, we cannot just delegate to another constructor.

[illuhad]

Are we sure we don't want external storage in any case (i.e. never copy input data to internal storage)? For the pointer-based constructors, we:

• Use external storage and write-back for non-const pointers
• Copy to internal storage and disable write-back for const pointers

[nilsfriess]

I think I just misunderstood what use_external_storage is supposed to be, I hope I got it right now

[illuhad]

Thanks, I think it's good now :)

I have only one minor nitpick: In the SYCL headers, we use CamelCase only for template arguments, and snake_case everywhere else, including type traits or constexpr variables - similarly to the C++ standard library.