Refactor inner Txs interface

[grarco]

Possible improvement of the way we access the section commitments of an inner tx

enum TxCommitmentRef {
    Id(TxCommitments),
    Hash(Hash),
    Index(usize),
}

impl TxCommitmentRef {
    fn get_commitments(
        &self,
        batch: impl IntoIterator<Item = &TxCommitments>,
    ) -> Option<&TxCommitments> {
        match self {
            Self::Id(commitments) => {
                // FIXME: check that `commitments` is present in `batch`?
                commitments
            }
            Self::Hash(commitments_hash) => batch.into_iter().find_map(|commitments| {
                (hash(commitments) == commitments_hash).then_some(commitments)
            }),
            Self::Index(index) => batch.into_iter().nth(index),
        }
    }
}

Originally posted by @sug0 in #3103 (comment)

[sug0]

Depends on #3103