rustdoc: run rustfmt on type definitions

[fitzgen]

When we have a lot of generic parameters and where clauses, it becomes unreadable in rustdoc pretty quickly.

For example:

[hanna-kruppe]

The two PRs backlinked above made me think that perhaps we should rustfmt all code-like snippets included in rustdoc's output. I am not 100% sure, since the snippets in API docs aren't always like real code and don't always serve the same function. But since it's generally supposed to look like Rust code, I guess it's reasonable to apply the same style to it as to real code.

[Mark-Simulacrum]

Today this looks much better. I think this is effectively "fixed".