Metadata extraction for Metadata Table View

[SlaunchaMan]

This started as an exercise to code-golf flattenXcodeProjects(_:targets:) after being poked by @nwest. I got this far:

private func flattenXcodeProjects(projects:[CPXcodeProject], targets:[CPCocoaPodsTarget]) -> [AnyObject] {
  return projects.flatMap { project in
    [project] + project.targets.flatMap { target -> [AnyObject] in
      let pods = target.cocoapodsTargets.flatMap { targetName in
        targets.filter { $0.name == targetName }.flatMap { $0.pods }
      } as [AnyObject]

      return [target] + pods + ["spacer"]
    }
  }
}

The more I used it, though, the more I realized that there are two things happening in this file: converting from a list of Xcode projects and a list of CocoaPods targets to an array of entries to display in a table view, and displaying metadata from that list to display in the table. Both of those tasks seemed well-suited to protocols. So, this PR creates two protocols: CPMetadataExtractable, which covers converting the two arrays into one array of metadata, and CPMetadataRepresentable, which covers displaying the data in the table.

This is a pretty big change, but it buys more type safety. Notice that tableView(_:heightOfRow:) and tableView(_: viewForTableColumn:row:) no longer need to handle the “else” case of displaying nothing, since the array of projectMetadataEntries is type-safe.

Overall, in exchange for a net gain of 14 lines, we’ve gained a lot of safety. I think it’s a net positive.

[orta]

Any idea if this has bad compiler performance characteristics? http://irace.me/swift-profiling/

[SlaunchaMan]

I haven't checked. I should have some time later today to do a comparison.

[nwest]

@SlaunchaMan do you mind merging master again?