Extract code into external files

[carols10cents]

Fixes #1889.

Can't be merged until rust-lang/mdBook#1003 is merged and released and the released version of mdbook is available in rust-lang/rust, and the following are completed:

TODO

• Switch from line numbers to anchors throughout
  • rg rustdoc_include.*\\.rs:[0-9] src/*.md
• Get tests passing using my fork of mdbook for the moment
• Write a script that extracts:
  • anchor comments
  • "snip" comments
  • empty fn main() {}s in src/lib.rss
    and produces a github "release" artifact that's just the listings to download
• Document the github releases somewhere, and what they correspond to (print, rust version, etc)
• Update the nostarch generation script to process the includes before concatenating the markdown

In future PRs (this is just for my own brain parking lot to keep me on task)

• generate and include the output of running programs too
• write a script to regenerate output
• run rustfmt on all listings
• make one script to run when we want to update the version of rust the book uses
• file and fix a bug where include doesn't work within blockquotes: Include directives don't like being within blockquotes  mdBook#1127

> ```rust,ignore,does_not_compile
> {{#include ../listings/ch05-using-structs-to-structure-related-data/no-listing-02-ownership-of-struct-data/src/main.rs}}
> ```

[steveklabnik]

Triage: mdbook PR is still open.

[carols10cents]

Closing in favor of a new PR I've organized differently.