rustdoc: improve sidebar, add to crate root page, add tooltips

- Add sidebar to crate root index.html, to preview the structure of
  the crate quickly.

  If there are too many documents in the crate root, such as log\
  index.html, you can't find any informations of its mods and types,
  unless and untill you scroll down to the bottom of the page.

- Add tooltips to links of sidebar, showing shortened doc-comments.


Preview:
http://liigo.com/tmp/rust/doc/std/index.html
http://liigo.com/tmp/rust/doc/log/index.html