Add Proto version of package interfaces

[johnynek]

part of #54

This adds proto serialization of interfaces, but does not yet use them.

The idea is to allow the CLI to depend on proto interfaces and avoid having to transitively recompile code in order to use dependencies. We should also emit proto interfaces when we compile. Lastly, we need to make the bazel rules aware of these so it can plumb them through correctly. That will be in follow up PRs.

[codecov-io]

Codecov Report

Merging #252 into master will decrease coverage by 0.37%.
The diff coverage is 72.04%.

@@            Coverage Diff             @@
##           master     #252      +/-   ##
==========================================
- Coverage      86%   85.62%   -0.38%     
==========================================
  Files          55       55              
  Lines        3758     3890     +132     
  Branches      151      156       +5     
==========================================
+ Hits         3232     3331      +99     
- Misses        526      559      +33

Impacted Files	Coverage Δ
...src/main/scala/org/bykn/bosatsu/ExportedName.scala	97.05% <ø> (ø)	⬆️
cli/src/main/scala/org/bykn/bosatsu/Main.scala	0% <0%> (ø)	⬆️
core/src/main/scala/org/bykn/bosatsu/Package.scala	98.38% <100%> (ø)	⬆️
...rc/main/scala/org/bykn/bosatsu/Normalization.scala	92.91% <100%> (ø)	⬆️
...e/src/main/scala/org/bykn/bosatsu/Identifier.scala	88.88% <100%> (+0.42%)	⬆️
...e/src/main/scala/org/bykn/bosatsu/Evaluation.scala	89.95% <100%> (ø)	⬆️
core/src/main/scala/org/bykn/bosatsu/Codegen.scala	63.55% <50%> (ø)	⬆️
...main/scala/org/bykn/bosatsu/TypedExprToProto.scala	79.45% <80.74%> (+1.19%)	⬆️
...e/src/main/scala/org/bykn/bosatsu/StringUtil.scala	96.66% <0%> (-1.67%)	⬇️
...rc/main/scala/org/bykn/bosatsu/TotalityCheck.scala	96.26% <0%> (-0.38%)	⬇️
... and 2 more

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