halo2_poseidon: Add no-std support #4079

Job	Run time
Bitrot check	1m 22s
Build target wasm32-unknown-unknown	1m 48s
Book tests	1m 28s
Build target wasm32-wasi	1m 41s
Test on i686-unknown-linux-gnu	13m 11s
Test on ubuntu-latest	7m 19s
Test on i686-unknown-linux-gnu with beta features	13m 28s
Test on windows-latest	11m 37s
Test on i686-unknown-linux-gnu with nightly features	15m 57s
Test on macOS-latest	6m 42s
Code coverage	3m 37s
Test on ubuntu-latest with beta features	7m 15s
Intra-doc links	1m 29s
Test on windows-latest with beta features	11m 1s
Test on macOS-latest with beta features	7m 1s
Rustfmt	15s
Test on ubuntu-latest with nightly features	8m 29s
Test on windows-latest with nightly features	11m 57s
Test on macOS-latest with nightly features	7m 22s
	2h 12m 59s

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