Vector map variant

[mdedetrich]

Change data structure of safe JObject from Map to a custom VectorMap implementation which preserves key ordering on insertion of elements

Still to do

• Specializations for 1 .. 6n length VectorMap (same as Map implementation)
• Benchmarks
• Override more methods for performance (i.e. builders)

[codecov-io]

Codecov Report

Merging #19 into master will decrease coverage by 2.18%.
The diff coverage is 23.68%.

@@            Coverage Diff            @@
##           master     #19      +/-   ##
=========================================
- Coverage   44.68%   42.5%   -2.19%     
=========================================
  Files           5       6       +1     
  Lines         687     760      +73     
  Branches      133     140       +7     
=========================================
+ Hits          307     323      +16     
- Misses        380     437      +57

Impacted Files	Coverage Δ
jvm/src/main/scala/scalajson/ast/JValue.scala	71.17% <ø> (-2.23%)	⬇️
...s/src/main/scala/scalajson/ast/unsafe/JValue.scala	0% <0%> (ø)	⬆️
js/src/main/scala/scalajson/ast/JValue.scala	0% <0%> (ø)	⬆️
...n/scala/scala/collection/immutable/VectorMap.scala	25.37% <25.37%> (ø)
...m/src/main/scala/scalajson/ast/unsafe/JValue.scala	59.5% <50%> (ø)	⬆️

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[gmethvin]

I think we need some tests for VectorMap itself.

Hopefully @SethTisue or someone else from the scala team can clarify, but is it appropriate to use the scala.collection package here?

Maybe we should keep the VectorMap class private for now and only use the Map or immutable.Map type in the public API.

[eed3si9n]

I would like to echo @gmethvin's concern here. I don't think we should use package scala here.

/cc @SethTisue

[SethTisue]

sorry I missed this before. while it's certainly possible this could eventually become part of the Scala 2.13 collections, for now it's not okay to use scala.collection