GROOVY-6554: Improved serialization performance of JsonOutput
Merge branch 'impr-json-output' of github.com:bura/groovy-core into bura-impr-json-output

Conflicts:
	subprojects/groovy-json/src/main/groovy/groovy/json/JsonOutput.groovy
	subprojects/groovy-json/src/test/groovy/groovy/json/JsonOutputTest.groovy