[boost] [compute] kernels as strings impairs readability and maintainability

Hi, While reading through the code of Boost.Compute to see what it does and how it does it, I often found that the approach used by the library of putting all OpenCL kernels inside of strings was an annoying limitation and made it quite difficult to reason with them, much less debug or maintain them. This has a negative effect on the effort needed to contribute to the library. Has separate compilation been considered? Put the OpenCL code into .cl files, and let the build system do whatever is needed to transform them into a form that can be executed. This would also make it easier to eventually provide a CUDA backend.