Change static member constraints name `^S` to `^U`

I apologise if this seems a bit nitpicky, but it initially confused me a bit on whether the actual members where defined on `State` or `Update` types, since the last function used `^S` for State and `^U` for `Update`, while the others used `^S` for `Update`.
I hope you don't mind, but I have changed the code to reflect that such that `^U` is used for all places where you expect an `Update`-like type.