Add tailRecM and MonadError instance for Decoder

[peterneyens]

I based the tailRecM and handleErrorWith implementations on the instances of Either and io.circe.Decoder.

[codecov-io]

Codecov Report

Merging #53 into master will increase coverage by 0.22%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
+ Coverage   90.31%   90.54%   +0.22%     
==========================================
  Files          12       12              
  Lines         289      296       +7     
  Branches        4        4              
==========================================
+ Hits          261      268       +7     
  Misses         28       28

Impacted Files	Coverage Δ
modules/cats/package.scala	95.83% <100%> (+1.71%)	⬆️

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[peterneyens]

I also have an Arrow instance for Decoder but I ran into typelevel/cats#1567, so we can either add it after that's fixed in the next Cats version or add it untested.

[andyscott]

Awesome, thank you, and LGTM!