feat: make it #[no_std] compatible

[th7nder]

This PR makes it #[no_std] compatible.
It was tested in a substrate pallet extrinsic and called in polkadot.js.org explorer:

        pub fn test_neptune(origin: OriginFor<T>) -> DispatchResult {
            let _caller = ensure_signed(origin)?;

            let constants: PoseidonConstants<Fr, U8> = PoseidonConstants::new();
            let mut preimage = [Fr::ZERO; 8];
            preimage[0] = <Fr as Field>::ONE;

            let mut h = Poseidon::<Fr, U8>::new_with_preimage(&preimage, &constants);
            let _aa = h.hash();

            Ok(())
        }

I used libm to make the ceil() and friends available in no_std environments, not sure whether this is the right call for every env.

[huitseeker]

Back to your queue for one linting fix, otherwise this LGTM. You can also set up the PR to allow maintainer modifications to your branch and we'll shepherd that through.