Optimize JSON object builder

[xerial]

Optimize JSONObject builder by directly pushing JSONValues into JSONContext (stack)

[codecov]

Codecov Report

Merging #216 into master will decrease coverage by 1.05%.
The diff coverage is 82.6%.

@@            Coverage Diff             @@
##           master     #216      +/-   ##
==========================================
- Coverage   83.71%   82.66%   -1.06%     
==========================================
  Files         129      131       +2     
  Lines        5067     5128      +61     
  Branches      520      519       -1     
==========================================
- Hits         4242     4239       -3     
- Misses        825      889      +64

Impacted Files	Coverage Δ
...shared/src/main/scala/wvlet/log/io/StopWatch.scala	80% <100%> (-3.34%)	⬇️
...json/src/main/scala/wvlet/airframe/json/JSON.scala	60.97% <100%> (+0.44%)	⬆️
...n/scala/wvlet/airframe/json/JSONEventHandler.scala	100% <100%> (ø)
...in/scala/wvlet/airframe/json/NullJSONContext.scala	72.72% <72.72%> (ø)
...c/main/scala/wvlet/airframe/json/JSONScanner.scala	80.65% <78.78%> (-1.26%)	⬇️
...n/scala/wvlet/airframe/json/JSONValueBuilder.scala	83.78% <83.78%> (ø)
... and 2 more

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