AndThen optimisation

[alexandru]

We used AndThen for the Sync[StateT] instance.

At this point its future in cats-effect is a little uncertain due to typelevel/cats#2187 - however if that PR doesn't make it in Cats, then it will probably continue to live on in cats-effect.

This PR introduces the same optimization used for IO#map, doing fusion in its andThen and compose up to a maximum threshold.

[codecov-io]

Codecov Report

Merging #146 into master will decrease coverage by 4.84%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #146      +/-   ##
==========================================
- Coverage   89.87%   85.02%   -4.85%     
==========================================
  Files          47       47              
  Lines         978     1042      +64     
  Branches       82       78       -4     
==========================================
+ Hits          879      886       +7     
- Misses         99      156      +57

[alexandru]

Will merge this, because AndThen is already on master and this optimization is benign.

Will re-evaluate if we need AndThen or not, once this goes through review: typelevel/cats#2187