Normative: access the .next method only once (#1021)

As discussed in #976, this PR changes the how iteration works in the spec. Previously, the next method would be fetched off the iterator object on each iteration. This PR changes that to happen exactly once at the start of the protocol.

Some places in the spec used to pass a Record around with the iterator and the completion status and others just pass the iterator object. This has been simplified to make every point of iteration pass a Record containing the iterator, completion status, and now the next method.