Incremental computation

I did a proof of concept of using DBSP for incremental view maintenance:

https://github.com/penberg/limbo/tree/incremental

However, we can apply DBSP much more broadly than that. For example, we could do query subscriptions:

conn.observe("SELECT * FROM users WHERE active = 1", |row| {
    let id: i32 = row.get("id").unwrap();
    let name: String = row.get("name").unwrap();
    println!("User {} with id {}", name, id);
});

but also probably triggers.

Although I wrote everything from scratch, we should probably just use the https://crates.io/crates/dbsp crate.