halo2_poseidon: Add no-std support #4077

Job	Run time
Book tests	1m 26s
Bitrot check	1m 25s
Build target wasm32-unknown-unknown	1m 51s
Code coverage	3m 18s
Intra-doc links	1m 30s
Build target wasm32-wasi	1m 39s
Rustfmt	14s
Test on ubuntu-latest	2m 55s
Test on i686-unknown-linux-gnu	2m 58s
Test on windows-latest	5m 42s
Test on i686-unknown-linux-gnu with beta features	3m 12s
Test on macOS-latest	2m 52s
Test on i686-unknown-linux-gnu with nightly features	3m 2s
Test on ubuntu-latest with beta features	2m 53s
Test on windows-latest with beta features	4m 25s
Test on macOS-latest with beta features	2m 26s
Test on ubuntu-latest with nightly features	2m 58s
Test on windows-latest with nightly features	5m 31s
Test on macOS-latest with nightly features	2m 53s
	53m 10s

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