enter_at is specialized to Product scopes

I might be missing something, but going directly from a top-level scope into an AltNeu sub-scope is a bit unergonomic at the moment, because enter_at assumes a Product timestamp. I'm haven't had the time to check where this specialisation is enforced.

In any case, it's not a big deal, I'm working around this by simply breaking out all of the stuff that enter_at is doing under the hood.

scope.scoped::<AltNeu<S::Timestamp>, _, _>("AltNeu", |subgraph| {
    let proposals = propose
        .as_collection(|e: &Value, _v| e.clone())
        .inner
        .enter(subgraph)
        .delay(move |_datum, t| AltNeu { neu: true, ..t.clone() })
        .map(move |(data, t, diff)| {
            let new_time = AltNeu { time: t.clone(), neu: true }
            (data, new_time, diff)
        });
});

Start by locating the enter_at implementation and where it assumes a Product timestamp; compare it with the scoped and enter paths shown in the issue. The work is done when a top-level scope can enter an AltNeu sub-scope without reimplementing the underlying operations.