Support `--sql` in comments for Rust

[mcmah309]

It would be nice if this worked

// --sql
r#"
SELECT EXISTS (
    SELECT (1, 1)
    FROM bins AS b
    JOIN group_bins AS gb ON gb.bin_id = b.id
    JOIN user_groups AS ug ON ug.group_id = gb.group_id
    WHERE b.id = :bin_id
        AND ug.user_id = :user_id
        AND gb.bins_addable = 1
) AS has_permission;"#;

Instead of

r#"--sql
SELECT EXISTS (
    SELECT (1, 1)
    FROM bins AS b
    JOIN group_bins AS gb ON gb.bin_id = b.id
    JOIN user_groups AS ug ON ug.group_id = gb.group_id
    WHERE b.id = :bin_id
        AND ug.user_id = :user_id
        AND gb.bins_addable = 1
) AS has_permission;"#;