cargo private

ConflictStoreTrie: Faster filtered search This is an optimization that I was thinking of doing in #6283. I did not then as this is optimizing a not particularly hot path. I wish to do it now as it is stuck in my head, and I need that head space for more important things. This also "accidentally" fixes the [indeterminacy](https://github.com/rust-lang/cargo/pull/6283#issuecomment-443358988) introduced in #6283, by replacing the `HashMap.iter().find()` like code with `BTreeMap.iter().find()` like code. This is not strictly needed, as @alexcrichton pointed out (In most use cases the index will change between any two invocations anyway), but does make it much easier to deal with (fuzz) test cases.