refactor: reverse order iterator for transaction queue

[ayeletstarkware]

Update TransactionQueue iter to return reverse order iterator.

• Refactored the iter function to return an iterator in reverse order, contrasting with the BTreeSet iterator which returns in ascending order.
• This change makes the iteration more intuitive for the mempool queue, which returns transactions by reverse tip order.

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[elintul]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @ayeletstarkware)

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crates/mempool/src/transaction_queue.rs line 43 at r1 (raw file):

    /// Gets an iterator that visits the elements in the queue in descending
    /// order, in contrast to the `BTreeSet` iterator which visits in ascending order.

We can use the same doc. as mempool's iter.
Note that referencing BTreeSet is beyond the scope here; focus on behavioral documentation, avoiding implementation details as much as possible.

Suggestion:

    /// Returns an iterator of the current eligible transactions for sequencing, ordered by their
    /// priority.

[elintul]

Reviewed 2 of 2 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @ayeletstarkware)