Implement Executor for double references

[jplatte]

So now I got something that compiles , but I actually think it's mostly useless since AFAICT one still can't write a function that's generic over connection / pool / transaction with this 😞

I think this enables

async fn do_stuff<E>(mut executor: E) -> sqlx::Result<()>
where
    &mut E: Executor<DB>,
{
    sqlx::query!("…").execute(&mut executor).await?;
    let foo = sqlx::query_as!(Foo, "…").fetch_one(&mut executor).await?;

    Ok(())
}

[jplatte]

I've tried it. While SQLx itself compiles with these changes, any way I've tried to use it results in overflow evaluating the requirement `&mut &mut _: sqlx::Executor<'_>` .

[mehcode]

Thanks for the attempt. We definitely need to revisit this when GATs are available, even in nightly.