RUST-1508 Implement conversion traits between strings and ObjectId

[clarkmcc]

I'd like to be able to do things like

fn query<K: TryInto<ObjectId>>(key: K) {}

fn main() {
    let key = "000000000000000000000000";
    query(key);

    let key = String::from("000000000000000000000000");
    query(key);

    let key = ObjectId::new();
    query(key);
}

but it doesn't seem like the necessary conversion traits have been implemented. I'm not a Rust expert and so I cannot speak to what the solution is, but it would be really convenient to be able to both pass ObjectIds and types that can be turned into ObjectIds.