Improve bcgen. #432

Job	Run time
docs	52s
host=ubuntu-22.04 cmd=cli target=x86_64-linux-gnu mode=release	2m 29s
host=ubuntu-22.04 cmd=cli target=x86_64-selinux-gnu mode=release	2m 20s
host=macos-12 cmd=cli target=x86_64-macos-none mode=release	2m 54s
host=macos-12 cmd=cli target=aarch64-macos-none mode=release	2m 15s
host=windows-2022 cmd=cli target=x86_64-windows-gnu mode=release	2m 46s
host=windows-2022 cmd=lib target=x86_64-windows-gnu mode=release	2m 46s
host=ubuntu-22.04 cmd=web-lib target=wasm32-freestanding mode=release	1m 53s
host=ubuntu-22.04 cmd=cli target=wasm32-wasi mode=release	1m 52s
host=ubuntu-22.04 cmd=lib target=x86_64-linux-gnu mode=release	1m 40s
host=macos-12 cmd=lib target=x86_64-macos-none mode=release	2m 2s
host=macos-12 cmd=lib target=aarch64-macos-none mode=release	1m 20s
Release	1s
	25m 10s

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