Implement caching mechanism for OpenCL

[athas]

Compiling the OpenCL program on every startup is very slow for large programs. Dumping a binary at first start and reloading it later is almost instant, but currently a manual process. It would be better if there was a C API function for telling a Futhark program to manage such caching in some specific file. The program would then automatically take care of cache invalidation and the like as needed. A similar feature can be implemented for the CUDA backend with cuModuleLoadData().