[ANN] type safe diffUTCTime

I feel like adding a way to get time inside of STM is kind of antithetical to what STM wants; STM wants to be able to freely (with minimal side effects) retry the entire block, but now there’s some outside function leaking in. I’m sure someone more knowledgeable will comment otherwise but that sticks out to me. I do enjoy the use of the type level nats to ensure one after the other, and a secondary library like this is the correct place to do that.