If you've been using the same iTunes account/Apple ID, you can go into the iTunes Store on your rMBP, find your purchases (should be somewhere on the right-hand side), and download the songs that are not already on your rMBP. Create your playlists, sync your new iPhone, and you're good to go.
There really isn't a non-clunky way of transferring music from an iPhone to a Mac. It was possible with regular iPods back in the day (I transferred most of my iTunes library from my family desktop to my personal MBP using my iPod Classic). For some reason Apple didn't make it possible with the iPhone.