As noted by Ben and Steve, this is a current limitation of embedded swift, but won't be forever!
I think I could use a buffer pointer here, but as noted later in the post the cost of redundant null checks actually adds up on this platform and similarly the bounds checks might have an appreciable effect on Conways performance.
I didn't consider using other packages because I was building with makefiles. I also am not sure if that package uses features not available in embedded mode.
I actually tried to make this, but couldn't get it to work with function pointers! Have you had success with dynamic member look up and closures/function pointers?