stringutil: emit multiple chunks when pretty printing This avoids concatenating output inside pprintgen() itself. But the real reason for this is it will make it easier to add indentation, as we'll need to account for indentation when emitting each individual object in a collection. The verbosity of this code compared to the original is a bit unfortunate. But I suppose this is the price to pay for having nice things (streaming and indenting). We could probably abstract the "print a collection" bits into a generic function to avoid some duplication. But I'm not overly inclined to do this. Differential Revision: https://phab.mercurial-scm.org/D4398