Auto merge of #24116 - zaeleus:rustdoc-codespan, r=alexcrichton

Because the current style for `code` in rustdoc is to prewrap whitespace, code spans that are hard wrapped in the source documentation are prematurely wrapped when rendered in HTML. [For example][2],

```
/// ...
/// type can be borrowed as multiple different types. In particular, `Vec<T>:
/// Borrow<Vec<T>>` and `Vec<T>: Borrow<[T]>`.
```

renders as

![screen shot 2015-04-06 at 12 11 21](https://cloud.githubusercontent.com/assets/191331/7008216/2706b3b0-dc56-11e4-941e-1b0154fcbc5c.png)

because "`Vec<T>: Borrow<Vec<T>>`" wraps to the next line in the source.

CommonMark 0.18 [[1]] specifies "interior spaces and line endings are collapsed into single spaces" for code spans, which would actually prevent this issue, but hoedown does not currently conform to the
CommonMark spec.

The added span-level callback attempts to adhere to how whitespace is handled as described by CommonMark, fixing the issue of early, unintentional wrapping of code spans in rendered HTML.

[1]: http://spec.commonmark.org/0.18/
[2]: https://doc.rust-lang.org/std/borrow/trait.Borrow.html