Add ability to try to get an input and remove an input

[jfecher]

Currently, the only way to retrieve an input in inc-complete is to get the result of its computation:

fn main() {
    let mut db = Db::<MyStorage>::default();
    db.update_input(MyInput(3), 6);
    let input = MyInput(3).get(db);
    assert_eq!(input, 6);
}

However, this will panic if the input was never set:

#[test]
#[should_panic]
fn no_such_input() {
    let mut db = Db::<MyStorage>::default();
    let _ = MyInput(3).get(db);
}

It would be convenient if there was a way to query whether an input exists:

#[test]
fn no_such_input2() {
    let mut db = Db::<MyStorage>::default();
    let input = MyInput(3).try_get(db);
    assert_eq!(input, None);
}

Additionally, there is no way to remove existing inputs currently. This can lead to unnecessary bloat over time, particularly when reading from pre-existing serialized dbs. A way to remove inputs would be helpful:

#[test]
fn remove_input() {
    let mut db = Db::<MyStorage>::default();
    let input = MyInput(3);
   
    assert_eq!(input.try_get(db), None);
    input.set(db, 6);
    assert_eq!(input.try_get(db), Some(6));
    input.remove(db);
    assert_eq!(input.try_get(db), None);
}