Accessor aliases

[ProGTX]

In order to reduce the verbosity of programming with SYCL accessors, this proposal aims to reduce the number of template parameters to just 2 from the current 5. It achieves this by slightly revising how to treat read-only data and adding alias templates based on the access target.

1. Main changes
   • Default accessor template parameters
   • Simplify access modes
   • Alias templates based on access target
   • Extend the handler class
2. Accessor alias templates
   • `constant_buffer_accessor
   • host_accessor
3. Treat const T the same as access::mode::read
   • Simplifies a lot of code
4. Define implicit conversions for equivalent types
5. Implicit conversions that add const
6. Overload for handler::require that also takes an access mode
   • To weaken the access mode
7. Default all accessor template parameters
   • Expect the data type
   • Default to an accessor to global buffer with read-write access
   • Assumes accessors can be a placeholder without template parameter
8. Discussed some considerations and alternatives
9. Examples of reduced verbosity

[ProGTX]

That looks like a good simplification. You could also add some CTAD if compiled with C++17 or higher.

@keryell I've greatly simplified the proposal, and I've added a section on CTAD. I think I would need to add deduction guides to the accessor class, I might still add that. There's also the problem that CTAD doesn't work on alias templates in C++17, so maybe I should add something for C++20 as well?

[keryell]

That looks good. Probably you need to coordinate with Intel to unify the simplifications they are pushing for too.

[ProGTX]

I've pushed revision 0.3, which focuses a lot more on CTAD and makes host_accessor a separate type