doc/style/changing directory is outdated.

[Havvy]

The directory https://github.com/rust-lang/rust/tree/master/src/doc/style/changing has a pre-1.0 page attached to it, and the post-1.0 page talks about higher kinded types.

Given that nobody's mentioned this yet, I don't think this chapter of the style guide is even useful, and should be fully axed.

[Havvy]

cc @steveklabnik

[steveklabnik]

We generally imported @aturon's stuff wholesale, and not a lot has been done with it. So yeah, I would imagine this section should probably just go.

[Havvy]

Closing in favor of PR.