[rustdoc] Replace module list items `ul`/`li` with `dl`/`dd`/`dt` elements
`@hywan` suggested that rustdoc should use `dl`,`dt` and `dd` HTML tags for listing items on module pages as it matches better what this is (an item and optionally its description). This is a very good idea so here is the implementation.
Also nice side-effect of this change: it reduces a bit the generated HTML since we go from:
This PR shouldn't impact page appearance.
```html
<ul class="item-table">
<li>
<div class="item-name">NAME</div>
<div class="desc docblock-short">THE DOC</div>
</li>
</ul>
```
to:
```html
<dl class="item-table">
<dt>NAME</dt>
<dd>THE DOC</dd>
</dl>
```
You can test it [here](https://rustdoc.crud.net/imperio/items-list/std/index.html).
r? `@notriddle`
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