Implementation of `PartialEq`, `Eq`, `PartialOrd` and `Ord` for `IndexMap` about indexmap

[cuviper]

IndexMap does implement equality in the same fashion as HashMap -- maps are considered equal if they contain equal keys and values. This is independent of order, which is natural for HashMap but perhaps surprising for IndexMap.

PartialOrd and Ord must agree with PartialEq and Eq in what they call Ordering::Equal, which means this comparison must also be independent of the IndexMap's order. It must also be antisymmetric and transitive, as well as being total for Ord. BTreeMap can easily implement a consistent comparison since the keys are sorted, but we have no such luxury here.

If you do know your keys are sorted, or if you just want to compare by each maps' current order regardless, you can iterate them and use Iterator::cmp and friends.

[torkleyy]

I see, thank you!