rustdoc: Add tooltips to sidebar (v3)

This pull request add tooltips to most links of sidebar.
The tooltips display "summary line" of items' document.

Some lengthy/annoying raw markdown code are eliminated, such as links and headers.
- `[Rust](http://rust-lang.org)` displays as `Rust` (no URLs)
- `# header` displays as `header` (no `#`s)

Some inline spans, e.g. ``` `code` ``` and ```*emphasis*```, are kept as they are, for better readable.

I've make sure `&` `'` `"` `<` and `>` are properly displayed in tooltips, for example, `&'a Option<T>`.

Online preview: http://liigo.com/tmp/tooltips/std/index.html

@alexcrichton @steveklabnik since you have reviewed my previous ([v1](https://github.com/rust-lang/rust/pull/13014),[v2](https://github.com/rust-lang/rust/pull/16448)) PRs of this serise, which have been closed for technical reasons. Thank you.