Bifoldable instance for Map

[Joseph C. Sible]

I'd like to propose a change to the containers package: adding a
Bifoldable instance to Map. I briefly mentioned this on Reddit [1] and
no obvious problems were brought up. I submitted a PR implementing it
[2]. This seems like an obvious and straightforward instance to me.
Thoughts?

Joseph C. Sible

[1]: https://old.reddit.com/r/haskell/comments/fsgqd6/monthly_hask_anything_april_2020/fn90d6k/
[2]: https://github.com/haskell/containers/pull/714