Refactor JObject structure #69

Job	Run time
analyze_jni	1m 27s
analyze_jnigen (stable)	42s
build_jni_example_android	4m 41s
build_jni_example_linux	1m 51s
build_jni_example_windows	2m 56s
run_pdfbox_example_linux	3m 14s
test_jnigen_windows_minimal	8m 16s
test_jnigen (ubuntu-latest, stable)	10m 0s
test_jnigen_macos_minimal	6m 45s
test_jnigen (ubuntu-latest, beta)	9m 58s
test_jni_macos_minimal	2m 31s
test_jni	1m 55s
test_jni_windows_minimal	4m 20s
coveralls_finish	5s
	58m 41s

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