Move kernels to their own files

This hopefully makes kernel development a lot easier: I've moved all the kernels (both OpenCL and CUDA) into their own files in the opencl/kernels/ and cuda/kernels/ directories, respectively, and update the code to simply load those files from source.

This was going to be necessary sooner or later anyway, since the kernel_src.hpp files were getting super unwieldy, and also because recompiling the entire bandicoot program shouldn't be necessary after a kernel is tweaked.

Now, one question raised by this is "how do we figure out where the kernel sources are?" For now, I've used a hack (just __FILE__), but eventually, since we will need some kind of on-disk configuration to cache precompiled kernels for various devices, we can just use that whenever it happens to appear.