Auto merge of #20221 - liigo:rustdoc-sidebar-tooltips-v3, r=alexcrichton

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.

src/librustdoc/html/static/main.js

@@ -668,6 +668,15 @@
             search();
         }
 
+        function plainSummaryLine(markdown) {
+            var str = markdown.replace(/\n/g, ' ')
+            str = str.replace(/'/g, "\'")
+            str = str.replace(/^#+? (.+?)/, "$1")
+            str = str.replace(/\[(.*?)\]\(.*?\)/g, "$1")
+            str = str.replace(/\[(.*?)\]\[.*?\]/g, "$1")
+            return str;
+        }
+
         index = buildIndex(rawSearchIndex);
         startSearch();
 
@@ -688,8 +697,10 @@
                 if (crates[i] == window.currentCrate) {
                     klass += ' current';
                 }
+                var desc = rawSearchIndex[crates[i]].items[0][3];
                 div.append($('<a>', {'href': '../' + crates[i] + '/index.html',
-                                    'class': klass}).text(crates[i]));
+                                     'title': plainSummaryLine(desc),
+                                     'class': klass}).text(crates[i]));
             }
             sidebar.append(div);
         }