rustdoc CSS does not reset line-height to a non-zero value in <pre>.

[eddyb]

Due to GitHub's strict limitations on HTML styling, one of the few ways I found to reduce font size has been the use of <sub>/<sup> tags. However, I recently stumbled upon a difference between GFM and rustdoc's theming:

on GitHub	on crates.io (rustdoc also identical)

To reproduce, this should be sufficient:

<sub>

```
one
two
three
```

</sub>

---

AFAICT, the difference is due to line-height: <sub>/<sup> set it to 0, and on GitHub we also have:

.markdown-body .highlight pre, .markdown-body pre {
  /* ... */
  line-height: 1.45;
  /* ... */
}

But rustdoc's normalize.css doesn't set line-height to anything in its code, kbd, pre, samp {...} rule.
I would suggest either line-height: normal or line-height: 1.15 (the latter is what it does for html).

(I hope crates.io uses rustdoc, otherwise this bug is basically duplicated to two uses of normalize.css)

[jyn514]

cc @notriddle, you like to work on similar issues I think