Consider implementing `std::ops` operators for owned `Zoned` types

[mqudsi]

Currently symbolic mathematical operations involving Zoned types are only implemented for &Zoned references (in part because Zoned is not Copy). This could lead one to assume that these operators are not defined/present at all when attempting to perform these operations owned Zoned instances (as would be most natural, for example, when dealing with temporaries).

This would be strictly in addition to the current definitions implemented on &Zoned references, to improve the usability of the library and make these features more discoverable (as originally discussed in #390).

(I'm not sure if there are any other types that this proposal should extend to, as I understand it all the remaining important types that one would perform math on are Copy?)

[BurntSushi]

Just to pluck out the salient point that convinced me here... The reason why these impls didn't already exist was because I was concerned about users consuming Zoned values unnecessarily (or cloning them unnecessarily) just to do some arithmetic with them. But I think this point, raised in #390, is true:

and for the cases where that's not the case, well, rust users are very used to going back and adding & before a value they logged or did something else with to prevent it from being consumed and extend its lifetime.

Plus, a clone isn't the end of the world in most cases in this context. It's just an Arc::clone.

(I'm not sure if there are any other types that this proposal should extend to, as I understand it all the remaining important types that one would perform math on are Copy?)

I think that's right. It was important to me that all primitive types in Jiff were Copy. The fact that Zoned isn't is, I suspect, an annoyance that will haunt for me for some time to come.