Add NonEmptyList#toNes (issue #2346)

[CucumisSativus]

The last missing pull request for #2346

[He-Pin]

Why.not toSet or toNonEmptySet

[kun-song]

@hepin1989 Beacuse there are already some codes where toNes refers to toNonEmptySet. In fact, this is confusing sometimes, beacuse it can be interperted as toNonEmptyStream too.

[CucumisSativus]

@hepin1989 Everything that @satansk + the fact I wanted to be consistent with the rest of naming from this issue where I implemented NonEmptyMap#toNel and NonEmptyList#toNem. I think that .toSet might because it does not mention that this set should be non empty.

If you guys think that this naming is misleading, i'm not really attached to it.

[codecov-io]

Codecov Report

Merging #2557 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2557      +/-   ##
==========================================
+ Coverage   95.14%   95.14%   +<.01%     
==========================================
  Files         361      361              
  Lines        6630     6634       +4     
  Branches      289      294       +5     
==========================================
+ Hits         6308     6312       +4     
  Misses        322      322

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/NonEmptyList.scala	100% <100%> (ø)	⬆️
core/src/main/scala/cats/data/EitherT.scala	97.9% <0%> (+0.04%)	⬆️

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[CucumisSativus]

Sorry for the mess in commits, screwed a bit with merging and then everything escalated quickly.

[CucumisSativus]

The build failed because of Java HotSpot(TM) 64-Bit Server VM warning: INFO: os::commit_memory(0x000000078ce4e000, 857415680, 0) failed; error='Cannot allocate memory' (errno=12). Could someone restart it?