Use std::execution::par_unseq in std::copy

[mitinarseny]

Use parallel unsequenced version of std::copy by providing std::execution::par_unseq execution policy tag as a first argument for std::copy function.
When I tried to use this policy, clang said:

/usr/lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/pstl/parallel_backend_tbb.h:19:10: error: 'tbb/blocked_range.h' file not found [clang-diagnostic-error]
#include <tbb/blocked_range.h>
         ^

As I googled the problem, it means that we should link to Intel TBB library in out CMakeLists.txt. But what if the library will be used for non-Intel processors?

[kovdan01]

As far as I know, execution policies are not yet properly supported by major compilers (GCC, Clang, MSVC) though it is C++17 (!) feature. So you just have to wait.