Fix long bug in error regions

[johnynek]

I wanted to merely improve error reporting by showing more than one type error when it was possible (#1044 )

Along the way, I found a long standing bug with locations incorrectly being set to be their outermost scope. I had noticed this before, but hadn't run it down. I had assumed it was part of typechecking and just due to inference being a bit non-specific when unifying across many types.

But in my example, I wound up printing out the Expr and seeing that the Declarations and Regions were set wrong.

The bug turned out to be the .as from Functor. The Expr traverse instance is doing what you say, but .as isn't what we meant. We wanted to only replace the outermost tag, not every tag transitively.

I have removed that code since we were only using it for this one bug. :)

This also should do more parallel checking of errors: if two adjacent lets don't depend on each other, we check in parallel.

We can do a bit better by only skipping lets that depend on a previous failed let, but I will punt that to a later PR.

[codecov-commenter]

Codecov Report

Patch coverage: 96.63% and project coverage change: +0.20% 🎉

Comparison is base (423a144) 91.75% compared to head (66405c8) 91.95%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1049      +/-   ##
==========================================
+ Coverage   91.75%   91.95%   +0.20%     
==========================================
  Files          92       93       +1     
  Lines        9549     9626      +77     
  Branches     2255     2258       +3     
==========================================
+ Hits         8762     8852      +90     
+ Misses        787      774      -13     

Files Changed	Coverage Δ
...re/src/main/scala/org/bykn/bosatsu/TypedExpr.scala	90.88% <ø> (+1.56%)	⬆️
core/src/main/scala/org/bykn/bosatsu/Expr.scala	89.52% <90.32%> (+1.68%)	⬆️
.../src/main/scala/org/bykn/bosatsu/rankn/Infer.scala	95.70% <98.21%> (+0.06%)	⬆️
...ore/src/main/scala/org/bykn/bosatsu/ListUtil.scala	100.00% <100.00%> (ø)
...src/main/scala/org/bykn/bosatsu/PackageError.scala	69.68% <100.00%> (+0.99%)	⬆️
.../main/scala/org/bykn/bosatsu/SourceConverter.scala	97.59% <100.00%> (+<0.01%)	⬆️

... and 8 files with indirect coverage changes

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