Add list literal constructors

[johnynek]

closes #13

This adds the construction, not pattern matching, for list literals.

There are two issues:

1. the parsing is still not correct since I can't parse function application in lists: [ Foo(a, b), c ] does not parse. I'm not sure why. It seems to be failing when it gets to the ,.
2. we effectively need macros here, but I have no way to be sure I reference a particular fully qualified function. It may be that we need to expand Expr to have point to imported names instead of just Var that refers to the current environment.

[codecov-io]

Codecov Report

Merging #91 into master will increase coverage by 0.92%.
The diff coverage is 91.95%.

@@            Coverage Diff             @@
##           master      #91      +/-   ##
==========================================
+ Coverage   69.23%   70.15%   +0.92%     
==========================================
  Files          36       37       +1     
  Lines        1885     1950      +65     
  Branches       42       43       +1     
==========================================
+ Hits         1305     1368      +63     
- Misses        580      582       +2

Impacted Files	Coverage Δ
core/src/main/scala/org/bykn/bosatsu/Codegen.scala	56.39% <0%> (-0.43%)	⬇️
core/src/main/scala/org/bykn/bosatsu/Package.scala	96.66% <100%> (+0.51%)	⬆️
...e/src/main/scala/org/bykn/bosatsu/Evaluation.scala	75.4% <100%> (+0.26%)	⬆️
.../src/main/scala/org/bykn/bosatsu/rankn/Infer.scala	85.56% <100%> (+0.05%)	⬆️
core/src/main/scala/org/bykn/bosatsu/Predef.scala	91.66% <100%> (+0.36%)	⬆️
...ore/src/main/scala/org/bykn/bosatsu/Referant.scala	100% <100%> (ø)	⬆️
core/src/main/scala/org/bykn/bosatsu/Parser.scala	82.02% <100%> (+0.62%)	⬆️
...ore/src/main/scala/org/bykn/bosatsu/ListLang.scala	100% <100%> (ø)
core/src/main/scala/org/bykn/bosatsu/Expr.scala	20.65% <33.33%> (ø)	⬆️
...re/src/main/scala/org/bykn/bosatsu/TypedExpr.scala	66.12% <83.33%> (ø)	⬆️
... and 2 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d9e040c...25197b1. Read the comment docs.