Simplify and unify free type variable handling

[johnynek]

This changes to require that all expressions being type checked by Infer have no free type variables. The caller needs to put the bounds in the correct places by using Generic(_, _). This is done in SourceConverter rather than spreading the logic around.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.06% ⚠️

Comparison is base (ec40f58) 91.80% compared to head (a927efe) 91.75%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1040      +/-   ##
==========================================
- Coverage   91.80%   91.75%   -0.06%     
==========================================
  Files          91       91              
  Lines        9487     9487              
  Branches     2172     2224      +52     
==========================================
- Hits         8710     8705       -5     
- Misses        777      782       +5     

Files Changed	Coverage Δ
core/src/main/scala/org/bykn/bosatsu/Expr.scala	87.14% <100.00%> (+0.57%)	⬆️
.../main/scala/org/bykn/bosatsu/SourceConverter.scala	97.59% <100.00%> (+<0.01%)	⬆️
.../src/main/scala/org/bykn/bosatsu/rankn/Infer.scala	95.79% <100.00%> (-0.05%)	⬇️

... and 6 files with indirect coverage changes

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