- Introduce diagrams into 17.05 to break up the wall of text and make it
easier to visualize.
- Extract the state machine code to make it easier to reference.
- Standardize on `await` instead of `.await` for inline code references.
- Move note about `race` and `select` up to 17.01 since the text now
references it there, and update the text in 17.03 to account for
already having introduced the function.
- Fix up a number of code samples to avoid showing `extern crate trpl;`.
- Rewrite the transition between 17.04 and 17.05 to account for having
restructured the chapter.
- Rework the example code in 17.05 to refer to the `page_title` example
from the rewritten 17.01 instead of the (now-removed) `hello`.
- When discussing the `Future::poll` method, explicitly address the `cx`
parameter: note what it is used for, and then move on.
