Improve error messages in DefRecursionCheck, Eval example

[johnynek]

I wanted to try implementing cats.Eval now that we have polymorphic recursion #1041 .

I couldn't see a way to do it without a poor version of the fuel pattern: promise up front how many recursions you would do at most.

But along the way, I noticed that we are only returning one error message from any branch in the code in DefRecursionCheck, so I implemented a version of the idea in #1044 which allowed me to see all the error messages at once (when they are on parts of the code I was already traversing).

[codecov-commenter]

Codecov Report

Patch coverage: 93.54% and project coverage change: +0.16% 🎉

Comparison is base (7890b7b) 91.81% compared to head (ccab783) 91.97%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1046      +/-   ##
==========================================
+ Coverage   91.81%   91.97%   +0.16%     
==========================================
  Files          91       92       +1     
  Lines        9533     9549      +16     
  Branches     2195     2255      +60     
==========================================
+ Hits         8753     8783      +30     
+ Misses        780      766      -14     

Files Changed	Coverage Δ
...in/scala/org/bykn/bosatsu/ParallelViaProduct.scala	85.71% <85.71%> (ø)
...ain/scala/org/bykn/bosatsu/DefRecursionCheck.scala	97.66% <95.83%> (-0.39%)	⬇️

... and 7 files with indirect coverage changes

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