Merge pull request #830 from zcash/halo2_poseidon-empty-crate #4057

Job	Run time
Bitrot check	1m 27s
Test on ubuntu-latest	1m 38s
Book tests	1m 22s
Test on windows-latest	1m 36s
Test on i686-unknown-linux-gnu	13m 37s
Test on i686-unknown-linux-gnu with beta features	13m 0s
Test on macOS-latest	1m 35s
Test on ubuntu-latest with beta features	1m 40s
Test on i686-unknown-linux-gnu with nightly features	15m 36s
Test on windows-latest with beta features	1m 36s
Test on macOS-latest with beta features	1m 40s
Build target wasm32-unknown-unknown	1m 46s
Test on ubuntu-latest with nightly features	1m 39s
Code coverage	3m 30s
Build target wasm32-wasi	1m 41s
Intra-doc links	1m 27s
Test on windows-latest with nightly features	1m 38s
Test on macOS-latest with nightly features	1m 36s
Rustfmt	13s
	1h 8m 17s

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