Replace Config field 5-tuple with a struct

[Rawk]

Add struct ConfigOption<T> to replace the 5-element tuple used for Config fields, to make the code more readable.

The tuple element 3 is actually true if the option is stable, the opposite of what the comment said.

Also changes IgnoreList::doc_hint to return "[<string>, ...]" instead of "[<string>,..]". It is now the same syntax as MacroSelectors::doc_hint.

[ytmimi]

@Rawk just a heads up, the team strongly discourages merge commits. No need to change that right now, but moving forward please rebase your changes in order to get your branches up to date.

[Rawk]

Thanks for the heads up. I have now rebased my recent pull request on master.

The default action was to merge, so i thought that was what i was supposed to do. I prefer to rebase.

You probably already know this, but there seems to be no way of specifying default action on GitHub. Instead, the master branch could set "Require linear history" here, and/or disable "Allow merge commits" here.

[ytmimi]

Awesome! Yeah, the team knows about those settings, and we'd definitely turn them on if we didn't need to allow merge commits for our subtree syncs with the rust-lang/rust repo.

[ytmimi]

@Rawk appreciate you trying to keep the PRs up to date, though rebasing the PRs every time there are new changes in master is a little unnecessary. Especially if you've got multiple PRs in flight at the same time since merging one will require you (or me, or another maintainer) to rebase the others anyway so we're just needlessly triggering CI.

Not an issue, but another thing I wanted to call out. Though if there are merge conflicts I totally understand rebasing to address those issues.