Channel history & Paginated Result API for Rest

[tiholic]

Rest#Channel#Channel#history will return PaginatedResult.
PaginatedResult supports methods: hasNext(), isLast(), async next() and async first() as per the docs#TG

void getHistory([ably.RestHistoryParams params]) async {
    // getting channel history, by passing or omitting the optional params
    var result = await channel.history(params);

    var messages = result.items; //get messages
    var hasNextPage = result.hasNext(); //tells whether there are more results
    if(hasNextPage){    
      result = await result.next();  //fetches next page results
      messages = result.items;
    }
    if(!hasNextPage){
      result = await result.first();  //fetches first page results
      messages = result.items;
    }
}

// history with default params
getHistory();

// sorted and filtered history
getHistory(
  ably.RestHistoryParams(
    direction: 'forwards',
    limit: 10,
    start: DateTime.fromMillisecondsSinceEpoch(0),
    end: DateTime.now()
  )
);

[QuintinWillison]

I've not looked at this pull request at the level of depth I would like to, but I'm short on time and we need to get a release out.

None of the comments I've made need block this pull request from landing so, @tiholic, please don't add any more commits to the underlying branch. For each of my comments, could you please either:

1. Create an issue to defer the work to fix it until later.
2. Create a pull request to fix it now if you have time (in which case that pull request's underlying branch should branch from the latest branch you've been working to allow the entire backlog to fold down correctly)

And then add a comment to the conversation to point towards which approach you'll take. I'll resolve the conversations later.

[QuintinWillison]

These @interface declarations and the corresponding @implementation blocks do nothing. I would prefer that we remove them as the inference is that they add value, but they don't.

[QuintinWillison]

For what it's worth, I think this code could benefit from some commentary. If nothing else for each of the three cases in the if statement. It's a little bit opaque on a brief scan. I'm finding myself puzzling over why _restHistory could be null, but also why you use ?/?? at the start but a direct comparison to == null later.

This is meant to be example code so we cannot have too much commentary. Many developers taking a look will be time poor so need their hands held.

[QuintinWillison]

Conflicts. 🤔 I'm resolving them now.