Add support for SYCL 2020 CTAD accessors

[psalz]

SYCL 2020 raised the required language level to C++17, and with that, several improvements to the existing interfaces could be made. One of the major usability improvements is to make it easier to create buffer accessors. Going from this:

auto acc = my_buffer.get_access<sycl::access::mode::read_write>(cgh);

To this:

sycl::accessor acc { my_buffer, cgh, sycl::read_write };

The most notable part of this change is the use of deduction tags such as sycl::read_write to infer the access mode using C++17 CTAD.

We should align our interfaces to match this, where possible. I'm creating this issue mostly for documentation purposes, as @facuMH is already looking into it.