Get list comprehensions working

[johnynek]

close #99

To solve this I had to fix a few lurking parser bugs, but the diff is small enough that I'm leaving it combined. There is no chance I don't want to have list/for comprehensions to match python.

They make several existing problems a bit more elegant and certainly looking more like python.

These are implemented basically as macros in the expansion from syntax (Declaration) into the lambda calculus based Expr. A more ambitious language might expose a means to make user defined macros which could do expansion into the Expr graph.

[codecov-io]

Codecov Report

Merging #174 into master will increase coverage by 0.25%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #174      +/-   ##
==========================================
+ Coverage   79.82%   80.07%   +0.25%     
==========================================
  Files          48       48              
  Lines        3112     3132      +20     
  Branches      119      129      +10     
==========================================
+ Hits         2484     2508      +24     
+ Misses        628      624       -4

Impacted Files	Coverage Δ
...ain/scala/org/bykn/bosatsu/DefRecursionCheck.scala	97.32% <ø> (-0.03%)	⬇️
core/src/main/scala/org/bykn/bosatsu/Pattern.scala	95.87% <ø> (ø)	⬆️
core/src/main/scala/org/bykn/bosatsu/Predef.scala	94.18% <100%> (+0.13%)	⬆️
.../src/main/scala/org/bykn/bosatsu/Declaration.scala	97.93% <100%> (+1.3%)	⬆️
core/src/main/scala/org/bykn/bosatsu/Parser.scala	91.32% <100%> (+0.74%)	⬆️
...ore/src/main/scala/org/bykn/bosatsu/ListLang.scala	100% <100%> (ø)	⬆️
...rc/main/scala/org/bykn/bosatsu/TotalityCheck.scala	95.71% <0%> (+0.38%)	⬆️

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