rustdoc: hardcode each header as a link.

This avoids having to include JS in the guide/tutorial/manual pages just
to get the headers being links. The on-hover behaviour showing the
little section marker § is preserved, because that gives a useful hint
that the heading is a link.

src/librustdoc/html/render.rs

@@ -1064,7 +1064,8 @@ fn item_module(w: &mut Writer, cx: &Context,
                 clean::ForeignStaticItem(..)   => ("ffi-statics", "Foreign Statics"),
                 clean::MacroItem(..)           => ("macros", "Macros"),
             };
-            try!(write!(w, "<h2 id='{}'>{}</h2>\n<table>", short, name));
+            try!(write!(w, "<h2 id='{id}'><a href=\"\\#{id}\">{name}</a></h2>\n<table>",
+                        id = short, name = name));
         }
 
         match myitem.inner {