Add From<rusqlite::Error> generic to conn functions

[tmpfs]

The rusqlite library has query_and_then which allows us to map rusqlite::Error to a higher-order Error type that implements From<rusqlite::Error>. This is particularly useful for loading rows and converting each row to a more application-specific type.

However, I can't then return those errors as async-sqlite is tightly bound to to rusqlite::Error in the conn functions.

If we could add another generic which requires E: From<rusqlite::Error> to the conn() functions then it would be possible to return custom errors and use query_and_then as intended.

Happy to submit a PR if you are open to this 🙏

[tmpfs]

Taking a quick look at this, it's possible to do:

    /// Invokes the provided function with a [`rusqlite::Connection`].
    pub async fn conn<F, T, E>(&self, func: F) -> Result<T, E>
    where
        F: FnOnce(&Connection) -> Result<T, E> + Send + 'static,
        E: From<rusqlite::Error> + From<Error> + Send + 'static,
        T: Send + 'static,
    {
        let (tx, rx) = oneshot::channel();
        self.conn_tx
            .send(Command::Func(Box::new(move |conn| {
                _ = tx.send(func(conn));
            })))
            .map_err(Error::from)?;
        Ok(rx.await.map_err(Error::from)??)
    }

But that breaks the conversion from plain rusqlite::Error due to the need for the additional From<Error> bound.

So perhaps the way to approach with would be with new methods conn_and_then and conn_mut_and_then - what do you think?

[tmpfs]

I've submitted #19 which solves the problem for me, just need to remember to specify the error type in the turbofish, for example:

        let rows = client
            .conn_and_then(move |conn| {
                let rows = /* load the rows */;
                Ok::<_, Error>(rows)
            })
            .await?;