Map and SortedMap should have MonoidK instances

[LukaJCB]

Map#++ is associative and should also form a Monoid with Map.empty as the identity :)

[kailuowang]

One thing to note is that this one needs to follow the MapInstancesBinCompat0 example - i.e.

1. Create a new MapInstancesBinCompat1 trait and add the instance there,
2. find all usages of MapInstancesBinCompat0 and add MapInstancesBinCompat1 there.
3. add 2 instance tests to cats.tests.MapSuite

[wojciechUrbanski]

I would like to take that one :)

[alexeygorobets]

Hey @wojciechUrbanski! Do you mind If I take this from you?

[wojciechUrbanski]

Hey @alexeygorobets. Sure, go on. Unfortunately i am a little bit occupied right now and cannot continue working on this one.

[alexeygorobets]

Thank you @wojciechUrbanski !

hi @kailuowang
Am I correct that I need to create a separate trait
trait AllInstancesBinCompat4 extends SortedMapInstancesBinCompat1 with MapInstancesBinCompat1
for maintaining binary compatibility?

[kailuowang]

@alexeygorobets that's correct. Thanks!

[rossabaker]

Fixed by #2744.