Fix Poseidon public api + small refactor

[jonas-lj]

• Ensure that the public api for Poseidon uses the wrapped type for BN254 field elements.
• Take references as inputs instead of consuming a vector.
• Move the zk-login specific poseidon hash function to zk_login.rs.

Note that the function used from sui-framework, poseidon_bytes is not changed, and in zklogin, there are no functional changes.