Add NonEmptyList#toNem (issue #2346)

[CucumisSativus]

Here comes the implementation of nel to nem, hope you like it :)

[LukaJCB]

Looks good, thanks!

[CucumisSativus]

Build for 2.11 failed because of:

Java HotSpot(TM) 64-Bit Server VM warning: INFO: os::commit_memory(0x000000077bc53000, 1144705024, 0) failed; error='Cannot allocate memory' (errno=12)

And I cannot rerun the build @LukaJCB could you help there?

[codecov-io]

Codecov Report

Merging #2541 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2541      +/-   ##
==========================================
+ Coverage   95.17%   95.17%   +<.01%     
==========================================
  Files         359      359              
  Lines        6549     6550       +1     
  Branches      278      273       -5     
==========================================
+ Hits         6233     6234       +1     
  Misses        316      316

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

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