Proposal: collection selection APIs

Originally posted to the Rust subreddit. Someone there suggested that I post it on IRLO as well. I hope this kind of "cross posting" is okay. I've been thinking about a potential new set of APIs for navigating and manipulating std collections, and I'd love some feedback on the idea. TL;DR: selections over a range of elements (with movable start/end) as a companion to the existing (but still unstable) cursors. The Rust standard library includes unstable cursor APIs for LinkedList (RFC, trackin...