Tagged adds

Old SPARC had an instruction which added or subtracted two numbers, but raised an exception if the low two bits of either input were nonzero or if the result overflowed. It could be an interesting research idea to implement something similar in silver or a risc-v extension (possibly in scope for the J working group, although said scope is ill defined) and get cakeml to use it. The difficult part is likely to be the trap handler and call back into wordLang code (against the normal order of transformations) to handle the instruction.