Support JSON Schema Draft 6 on `test` and `validate` #98

Job	Run time
docker	1m 29s
test (ubuntu-latest, clang, clang++, sh)	1m 6s
test (ubuntu-latest, gcc, g++, sh)	1m 10s
test (macos-latest, clang, clang++, sh)	46s
test (windows-latest, pwsh)	3m 18s
	7m 49s

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