Consider adding `impl TryFrom<Fact> for String` about biscuit-rust

[wez]

I've thrown together a couple of helper functions to make this a little more ergonomic in my application, sharing here in case it is helpful, and/or influences the query API here in this crate.

I'm sorting the results because that matters for determinism and matching in tests, but I can see how that may not always be 100% desirable.

let hostnames: Vec<String> =
    query_facts_scalar(&mut a, "data($name) <- hostname($name)")?;

/// Query facts and return them in a stable order.
fn query_facts<T, R, E>(authorizer: &mut Authorizer, query: R) -> Result<Vec<T>, Token>
where
    R: TryInto<Rule>,
    T: TryFrom<Fact, Error = E> + Ord,
    E: Into<Token>,
    Token: From<<R as TryInto<Rule>>::Error>,
{
    let mut data = authorizer.query(query)?;
    data.sort();
    Ok(data)
}

/// Query a set of scalar facts and return them in a stable order.
fn query_facts_scalar<T, R, E>(authorizer: &mut Authorizer, query: R) -> Result<Vec<T>, Token>
where
    R: TryInto<Rule>,
    E: Into<Token>,
    Token: From<<R as TryInto<Rule>>::Error>,
    (T,): TryFrom<Fact, Error = E> + Ord,
{
    let mut data: Vec<(T,)> = query_facts(authorizer, query)?;
    let data: Vec<T> = data.into_iter().map(|t| t.0).collect();
    Ok(data)
}