Bring sycl backends in line with the Cpp Core Guidelines

[kris-rowe]

Details

• http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines.html

Todo

This list is not comprehensive

• Use unnamed namespace instead of `static for local helper functions (F22)
• Eliminate output parameters by returning in a tuple with the error flag (F21)
• Add not_null where appropriate (F23)