Make Rebindable @trusted.

Maybe someone can come up with a reason why this isn't valid, but as far as I can tell, there's nothing unsafe going on here (particularly since `Rebindable` can't hold structs and thus have to deal with overloaded `opAssign`s and whatnot), and it's proving to be very annoying to add `@safe` to std.datetime with `Rebindable` being `@system`. So, if possible, I'd very much like to make this change.