Modify `DecoratorIterator` to return `&Decorator` instead of `DecoratorId`

[plafer]

Originally posted by @bobbinth in https://github.com/0xPolygonMiden/miden-vm/pull/1466#discussion_r1744607413

Maybe not for this PR, but I wonder if we can modify the DecoratorIterator to return the actual decorators rather than decorator IDs. If this works, we won't need to pass in the MastForest here.