Support polymorphic recursion

[johnynek]

This make monomorphization very hard (if not impossible) but removing this if we decide we really want monomorphization can be done.

In the mean time, I'm interested in exploring more powerful recursions that still provable terminate, and some examples I've hit require polymorphic recursion.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.09% 🎉

Comparison is base (dd86eea) 91.74% compared to head (d43c6b7) 91.83%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1041      +/-   ##
==========================================
+ Coverage   91.74%   91.83%   +0.09%     
==========================================
  Files          91       91              
  Lines        9487     9496       +9     
  Branches     2224     2241      +17     
==========================================
+ Hits         8704     8721      +17     
+ Misses        783      775       -8     

Files Changed	Coverage Δ
core/src/main/scala/org/bykn/bosatsu/Expr.scala	87.83% <100.00%> (+0.69%)	⬆️
.../main/scala/org/bykn/bosatsu/SourceConverter.scala	97.59% <100.00%> (ø)
.../src/main/scala/org/bykn/bosatsu/rankn/Infer.scala	95.63% <100.00%> (-0.17%)	⬇️

... and 7 files with indirect coverage changes

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