Ensure no meta loops in Infer

[johnynek]

I don't think loops in the TyMeta graph actually cause a problem, but avoiding them isn't difficult, and makes things a bit simpler to reason about.

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (ed5e2ff) 91.97% compared to head (9863beb) 92.22%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1064      +/-   ##
==========================================
+ Coverage   91.97%   92.22%   +0.24%     
==========================================
  Files          93       93              
  Lines        9881     9888       +7     
  Branches     2322     2335      +13     
==========================================
+ Hits         9088     9119      +31     
+ Misses        793      769      -24     

Files	Coverage Δ
.../src/main/scala/org/bykn/bosatsu/rankn/Infer.scala	96.87% <93.75%> (+0.04%)	⬆️

... and 6 files with indirect coverage changes

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