Implement IntoIterator for RpoDigest

[plafer]

The primary purpose of this change is to be able to move an RpoDigest into a vector (without copy). This is especially useful when loading hashes into the vm's StackInputs.

let digest = Digest::default();
let vec = Vec::new();

vec.extend(digest);

Compare with our 2 suboptimal choices prior to this change:

let digest = Digest::default();
let vec = Vec::new();

// 1. one-liner, but requires a copy
{
    vec.append(digest.as_elements().to_vec());
}

// 2. Require an extra variable to perform a move
{
    let digest: [Felt; 4] = digest.into();
    vec.extend(digest);
}

[sonarqubecloud]

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[bobbinth]

Looks good! Thank you!