Make SimpleQuote final

[eltoder]

It's not designed for inheritance, but it is used a lot and making it final will allow compiler to devirtualize some of the calls.

[coveralls]

coverage: 73.272%. remained the same
when pulling c587544 on eltoder:feature/simple-quote-final
into 6d6e7a7 on lballabio:master.

[lballabio]

Are you measuring any improvements? From inside a Handle<Quote>, which is the most common case and which contains a shared_ptr<Quote>, the compiler would not be able to devirtualize anyway. And when one stores a SimpleQuote, it's usually to call setValue which is already not virtual.

[eltoder]

I think this will allow to devirtualize destructor calls in many cases, including when holding shared_ptr<SimpleQuote>. I'll set something up to confirm.

[github-actions]

This PR was automatically marked as stale because it has been open 60 days with no activity. Remove stale label or comment, or this will be closed in two weeks.

[eltoder]

I looked into it. The improvement is limited to cases where we hold a shared_ptr<SimpleQuote> and call value() or isValid(), which is fairly rare.

The only other upside is that when getting a SimpleQuote from outside, we'll be certain that value() returns exactly the value that was passed into setValue(), but that is an even less common use case.

[lballabio]

Ok, thanks!