Add .toOptionT to option syntax

[marcodippy]

just a postfix version of OptionT.fromOption

[codecov-io]

Codecov Report

Merging #2253 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2253      +/-   ##
==========================================
+ Coverage   94.96%   94.96%   +<.01%     
==========================================
  Files         333      333              
  Lines        5797     5798       +1     
  Branches      221      216       -5     
==========================================
+ Hits         5505     5506       +1     
  Misses        292      292

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

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[ceedubs]

We already have a toEitherT on Either, so it seems consistent to me to have this.