[ffigen] Retain listener blocks before invocation (#1574) #1

Job	Run time
build_jni_example_linux	1m 42s
analyze_jni	2m 2s
analyze_jnigen (stable)	1m 50s
build_jni_example_windows	4m 36s
build_jni_example_android	4m 31s
run_pdfbox_example_linux	2m 52s
test_jnigen_windows_minimal	5m 2s
test_jnigen_macos_minimal	3m 54s
test_jnigen (ubuntu-latest, stable)	7m 50s
test_jnigen (ubuntu-latest, beta)	9m 32s
test_jni_windows_minimal	4m 26s
test_jni	2m 1s
test_jni_macos_minimal	2m 53s
coveralls_finish	3s
	53m 14s

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