Kokkos execution spaces API review

[masterleinad]

Part of #7319. See kokkos/kokkos-core-wiki#581, https://kokkos.org/kokkos-core-wiki/API/core/KokkosConcepts.html?highlight=concepts#the-executionspace-concept, and https://kokkos.org/kokkos-core-wiki/API/core/execution_spaces.html#synopsis.

// This is not an actual class, it just describes the concept in shorthand
class ExecutionSpaceConcept {
public:
    typedef ExecutionSpaceConcept execution_space;
    typedef ... memory_space;
    typedef Device<execution_space, memory_space> device_type;
    typedef ... scratch_memory_space;
    typedef ... array_layout;
    typedef ... size_type;

    ExecutionSpaceConcept();
    ExecutionSpaceConcept(const ExecutionSpaceConcept& src);

    const char* name() const;
    void print_configuration(std::ostream ostr&) const;
    void print_configuration(std::ostream ostr&, bool details) const;

    int concurrency() const;

    void fence(const std::string&) const;
    void fence() const;

    friend bool operator==(const execution_space& lhs, const execution_space& rhs);
    friend bool operator!=(const execution_space& lhs, const execution_space& rhs);
};

template<class MS>
struct is_execution_space {
enum { value = false };
};

template<>
struct is_execution_space<ExecutionSpaceConcept> {
enum { value = true };
};

Backend-specific members:

Serial::Serial(NewInstance)
OpenMP::OpenMP(int pool_size)
HPX::HPX(hpx::execution::experimental::unique_any_sender<> &&sender)
HPX::get_sender()
Cuda::Cuda(cudaStream_t stream)
Cuda::cuda_stream()
Cuda::cuda_device()
Cuda::cuda_device_prop()
HIP(hipStream_t stream)
HIP::hip_stream()
HIP::hip_device()
HIP::hip_device_prop()
SYCL::SYCL(const sycl::queue&)
SYCL::sycl_queue() 
OpenACC(int async_arg)
OpenACC::acc_async_queue()
OpenACC::acc_device_number()