`OpenJtalk`を`Synthesizer<OpenJtalk> | Synthesizer<()>`として持つ #2317

Job	Run time
rust-lint	2m 59s
c-header	48s
actionlint	15s
rust-integration-test-strategy-matrix	1s
shellcheck	12s
validate-cargo-lock	20s
rust-unit-test	3m 6s
build-and-test-java-api (windows-latest)	6m 11s
build-and-test-java-api (macos-latest)	5m 56s
build-and-test-java-api (ubuntu-latest)	2m 48s
build-and-test-python-api (windows-latest)	6m 57s
build-and-test-python-api (macos-latest)	6m 57s
build-and-test-python-api (ubuntu-latest)	2m 40s
build-unix-cpp-example (macos-latest, osx-x64-cpu-cpp-shared)	2m 40s
build-unix-cpp-example (ubuntu-latest, linux-x64-cpu-cpp-shared)	1m 44s
build-windows-cpp-example	4m 29s
rust-integration-test (windows-2019, directml)	3m 26s
rust-integration-test (macos-11)	2m 42s
rust-integration-test (ubuntu-20.04)	1m 8s
	55m 19s

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