fn merge(&mut self, other: ComputedValue<'g>) {
        for (features, details) in other.target_inner {
            self.target_inner
                .entry(features)
                .or_default()
                .extend(details);
        }
        for (features, details) in other.host_inner {
            self.host_inner.entry(features).or_default().extend(details);
        }
    }
Adds all the instances in `other` to `self`.
tools/hakari/src/hakari.rs