Post #1629770

Despite Rust deliberately making your life miserable if you dare attempt to use unstable features that have existed for years, I caved and used the `gen_blocks` feature (https://github.com/rust-lang/rfcs/pull/3513) to reimplement a complicated iterator whose logic I had initially written by hand. They should really finish sanding the edges and stabilize this feature so that nobody needs to write iterators as explicit state machines ever again.