Rewire how kernel optimizations work #258

• Currently it uses get_cached_executor(), which branches on OPT_ENABLED, but we need to move to actx.call_loopy.
• Move to transform_loopy_program.
• Need to move e.g. register_optimization_preambles to the array context somehow too. The kernels no longer have a context or device, so they can't do that. Probably need to move this to actx.transform_loopy_program somehow?

Left over from #256.

cc @alexfikl