Relax callback bounds

[adwhit]

I noticed bounds on several of the traits are overly restrictive - they receive callbacks of type Fn when type FnOnce (or FnMut) would work. In general it is preferable to use the less restrictive type in APIs.

According to the docs, "Since both Fn and FnMut are subtraits of FnOnce, any instance of Fn or FnMut can be used where a FnOnce is expected." - therefore relaxing the trait bounds is non-breaking.

[charypar]

Thanks for doing this! ❤️ Looks good to me, but would like at least one more ✅.

Can't quite see whether we might want to restrict them again later or why we would. All those Fns are generally meant to be the event variant constructors which are used as a form of callback, so most of them are indeed called once.

The ones used in subscriptions get cloned because they potentially move between threads anyway (would be nice to avoid that, but I think I once tried and couldn't work it out)

[StuartHarris]

Yeah, personally, I think this is the right thing to do! Thank you, Alex! Couple of small changes to allow the examples to compile.

[StuartHarris]

Thanks!