Ch. 17: Add a new §05 on Stream and AsyncIterator

This pushes back the Tasks, Futures, and Threads discussion one more
spot, to become §06.

src/SUMMARY.md

@@ -106,7 +106,8 @@
     - [Concurrency With Async](ch17-02-concurrency-with-async.md)
     - [Working With More Futures](ch17-03-more-futures.md)
     - [More Ways of Combining Futures](ch17-04-more-ways-of-combining-futures.md)
-    - [Futures, Tasks, and Threads](ch17-05-futures-tasks-threads.md)
+    - [Streams and Async Iterators](ch17-05-streams-async-iterators.md)
+    - [Futures, Tasks, and Threads](ch17-06-futures-tasks-threads.md)
 
 - [Object Oriented Programming Features of Rust](ch18-00-oop.md)
     - [Characteristics of Object-Oriented Languages](ch18-01-what-is-oo.md)

src/ch17-04-more-ways-of-combining-futures.md

@@ -294,4 +294,6 @@ example, you can use this same approach to combine timeouts with retries, and
 in turn use those with things like network calls—the exact example we started
 out with at the beginning of the chapter!
 
+Up next, let’s look at how we can work with *sequences* of futures.
+
 [futures]: ch17-01-futures-and-syntax.html#what-are-futures

src/ch17-05-streams-async-iterators.md

@@ -0,0 +1,13 @@
+## Streams and Async Iterators
+
+<!--
+
+- Motivation: you can do a lot with `while let` but it would be nice to be able
+  to use `for` loops, even `async for`. (But we don’t get those from the
+  ecosystem traits… so maybe only call them out in a `Note: …` context?)
+
+- The basic API, with `poll_next()`, plus the “surface” syntax, `.next().await`.
+
+- How to use it, with a worked example from .
+
+-->

src/ch17-06-futures-tasks-threads.md

@@ -0,0 +1 @@
+# Futures, Tasks, and Threads