Get `&mut Map` back from entry API

Doesn't a simple if let else cover this?

    let a_or_b = if let a @ Some(_) = map.get_mut("a") {
        a
    } else {
        map.get_mut("b")
    };

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A method like Entry::or_get(alternative_key) for expressing this succinctly in cases where you are already using the Entry API would be nice to have, that's for sure.