Started by working on adding Word8Array sub/update simd functions, ended up mostly working on further integrating simd types into ssa and rssa

I added the word8array simd functions to prim.sig/fun

I corrected a bunch of compiler warnings about missing cases in various files in ssa and the backend

However I'm a bit stuck on an error from packed representation which is along the lines of
expects SimdRealSize.t but got Tycon.SimdRealSize.t, which is a bit odd since they should be the same. I poked around in prim-tycons and made a few changes
because simd types don't fit the established template since they have 2 size parameters. But I'm still getting that error, I'll look at it mor tomorrow.