Plonky3 MultiField32Challenger: transcript malleability and challenge entropy loss

Impact - Key: challenger/src/multifieldchallenger.rs | MultiField32Challenger::duplexing | transcriptmalleability - Affected files: challenger/src/multifieldchallenger.rs, field/src/helpers.rs - Violated invariant: The Fiat-Shamir sponge must bind challenges to the exact sequence of observed fiel...