[SYCL] Refactor handler to improve ABI stability (#987)

* Move some handler bits to the library
* Use SYCL containers
* Eliminate usage of kernel_impl inside handler
* Improve existing docs
* Reorder methods to exclude implementation details from public scope
* Move fields to the bottom to match rest of code

Signed-off-by: Alexander Batashev <[email protected]>

Author: Alexander Batashev

sycl/include/CL/sycl/event.hpp

@@ -9,6 +9,7 @@
 #pragma once
 
 #include <CL/sycl/detail/common.hpp>
+#include <CL/sycl/info/info_desc.hpp>
 #include <CL/sycl/stl.hpp>
 
 #include <memory>