Add all unstable features to Unstable Book.

[frewsxcv]

Add all unstable features to the Unstable Book, also remove a few that
either no longer exist or were promoted to stable.

These changes were extracted out of
#40694

[frewsxcv]

r? @steveklabnik

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @GuillaumeGomez (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[steveklabnik]

https://travis-ci.org/rust-lang/rust/jobs/214510619#L1351 ....... wat

[frewsxcv]

This should be passing now. The issue was that I was missing a file linked from the SUMMARY.md so mdbook was trying to create it, which caused the readonly error. This should be fixed now.

r? @steveklabnik

[steveklabnik]

The issue was that I was missing a file linked from the SUMMARY.md so mdbook was trying to create it, which caused the readonly error.

Oh nooo!

[steveklabnik]

@bors: r+ rollup

thanks 😄

[bors]

📌 Commit 589e01f has been approved by steveklabnik