Replace ::cl::sycl with SYCL 2020 ::sycl namespace

[psalz]

SYCL 2020 introduces a new canonical namespace ::sycl. While the old ::cl::sycl namespace remains available for backwards compatibility, there is no reason for us to keep using it. The only question is whether we want to do a single bulk update of all namespaces (somewhat polluting the git history), or transition gradually by using the new namespace in all new (and modified) code going forward.

[psalz]

This is less pressing now that we expose our own names through the ::celerity namespace and including the Celerity header also exposes the ::sycl namespace (#58). This means all that's left to do is to migrate our internal uses of the cl::sycl namespace.