0f8343830b
Stabilize Iterator::flatten in 1.29, fixes #48213. This PR stabilizes [`Iterator::flatten`](https://doc.rust-lang.org/nightly/std/iter/trait.Iterator.html#method.flatten) in *version 1.29* (1.28 goes to beta in 10 days, I don't think there's enough time to land it in that time, but let's see...). Tracking issue is: #48213. cc @bluss re. itertools. r? @SimonSapin ping @pietroalbini -- let's do a crater run when this passes CI :)