Add OptionOps.toInvalidNec and OptionOps.toValidNec

[ybasket]

We do have toInvalidNel and toValidNel already - just the the methods to lift an Option to a ValidatedNec are missing - though they come in very handy sometimes.

Complements the work already done #2475 and #2539.

[codecov-io]

Codecov Report

Merging #2555 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #2555      +/-   ##
=========================================
+ Coverage    95.2%   95.2%   +<.01%     
=========================================
  Files         361     361              
  Lines        6584    6586       +2     
  Branches      282     282              
=========================================
+ Hits         6268    6270       +2     
  Misses        316     316

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

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[LukaJCB]

Looks great, thank you! :)

[ceedubs]

Thanks @ybasket!