Use std::byte where appropriate

[bernhardmgruber]

We have a lot of places in alpaka where we deal with a region of bytes. E.g. are buffers, views, memset, etc. Since C++17 we can use the dedicated type std::byte to communicate the intent that we are dealing with raw bytes and not integers intended for computations. We should thus refactor the code and change types to std::byte where we really mean just a region of storage.