[KHR] Introduce "sycl free function kernels"

[koparasy]

This PR proposes a new KHR extension, sycl_khr_free_function_kernels, which lets an ordinary namespace-scope C++ function serve as a device kernel entry point.

A free function kernel is declared by decorating the function with a macro.

Its kernel arguments are the function's parameters (each must be device-copyable), giving them a defined order, and the kernel is identified by the function itself. As such it can be launched and queried by reference to the function rather than only as a lambda.

The extension defines:

• the decoration macros and the rules a free function kernel must obey;
• compile-time traits (is_nd_kernel_v<Func, Dims>, is_task_kernel_v<Func>) that report a function's kernel kind from its declaration alone;
• launch APIs (launch_grouped, launch_task) that take the kernel via a kernel_function<Func> handle.

[koparasy]

I am attaching the presentation that I did today.
Kernel Free functions KHR - Khronos Presentation.pdf

[TApplencourt]

Thanks a lot! I forwarded it to a few people.