Implement ToSql for Box<str>

[guissalustiano]

I'm using Box to use less space on heap and would like to write it direct to the database with diesel

[lsunsi]

@guissalustiano according to #4613 you might be missing some spots (:

[weiznich]

Yes it would be great to have it for the other Text like types as well and not only for Citext. Also some tests are necessary before accepting this.

[guissalustiano]

Thanks for the return, I'll implement this!

[LucaCappelletti94]

It is possible that PR #4610 will superseed this one as, following one of the PR change requests, I have added a blanket implementation for theToSql and FromSql traits for the Box struct.

[guissalustiano]

I didn't see that, thanks @LucaCappelletti94

[LucaCappelletti94]

I suggest to not close this PR until @weiznich says so, as it is still possible that there are issues with my own PR. Still, I just thought it important to comment that the two PRs are in collision for the implementation of traits for the Box.

[weiznich]

#4610 is slightly different than this PR. With what's currently implemented in that other PR it should be possible to load/store values from any Arc<T>/Rc<T>/Box<T> field where the T implements FromSql or ToSql.

At least for T == str and FromSql that's not the case yet, as you cannot create a unboxed (or arc/rc) str value out of nowhere. So in the end both PR's are somewhat required to address both slightly different uses-cases. That written it totally makes sense to support Rc<str> and Arc<str> here as well.

So yes, literally the change in this PR is made obsolete by the other PR but there are similar features that might still be useful to add.