Remove Order constraint from Hash instance for Map

[travisbrown]

This PR includes the commits by @vasiliybondarenko and @larsrh from #3060 but cleans up some of the deprecation messages, and also uses an override instead of a new name with a 1 suffix. I've also added a few lines of documentation explaining why the horrible override stuff in cats.instances.sortedSet is necessary.

[codecov-io]

Codecov Report

Merging #3126 into master will decrease coverage by 0.02%.
The diff coverage is 28.57%.

@@            Coverage Diff             @@
##           master    #3126      +/-   ##
==========================================
- Coverage   93.22%   93.19%   -0.03%     
==========================================
  Files         375      375              
  Lines        7291     7294       +3     
  Branches      196      192       -4     
==========================================
+ Hits         6797     6798       +1     
- Misses        494      496       +2

Flag	Coverage Δ
#scala_version_212	93.53% <28.57%> (ø)	⬆️
#scala_version_213	90.85% <28.57%> (-0.03%)	⬇️

Impacted Files	Coverage Δ
core/src/main/scala/cats/instances/sortedSet.scala	85.71% <25%> (-2.53%)	⬇️
...ala/cats/kernel/instances/SortedSetInstances.scala	91.66% <33.33%> (-8.34%)	⬇️
core/src/main/scala/cats/data/NonEmptyList.scala	99.35% <0%> (+0.64%)	⬆️

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[LukaJCB]

👍 thanks!