refactor: `Synthesizer`の実装を`Inner<_, A: Async>`の形にする #2765

Job	Run time
config	1s
deploy_model	7s
build_and_deploy (windows-2019, x86_64-pc-windows-msvc, windows-x64, plain-cdylib, true)	6m 8s
build_and_deploy (ubuntu-20.04, x86_64-unknown-linux-gnu, linux-x64, plain-cdylib, true)	2m 20s
build_and_deploy (macos-12, aarch64-apple-darwin, osx-arm64, plain-cdylib, true)	6m 37s
build_xcframework	0s
build_java_package	0s
download_test	0s
	15m 13s

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