rustdoc: Generate documentation for foreign items

This slurps up everything inside of an 'extern' block into the enclosing module
in order to document them. The documentation must be on the items themselves,
and they'll show up next to everything else on the module index pages.

Closes #5953

src/librustdoc/doctree.rs

@@ -30,6 +30,7 @@ pub struct Module {
     traits: ~[Trait],
     vis: ast::visibility,
     impls: ~[Impl],
+    foreigns: ~[ast::foreign_mod],
     view_items: ~[ast::view_item],
 }
 
@@ -50,6 +51,7 @@ impl Module {
             traits     : ~[],
             impls      : ~[],
             view_items : ~[],
+            foreigns   : ~[],
         }
     }
 }