rustdoc: Include source files with documentation

All items have source links back to their actual code. Source files can be
omitted with the doc(html_no_source) attribute on the crate. Currently there is
no syntax highlighting, but that will come with syntax highlighting with all
other snippets.

Closes #2072

src/librustdoc/rustdoc.rs

@@ -34,10 +34,11 @@ pub mod core;
 pub mod doctree;
 pub mod fold;
 pub mod html {
-    pub mod render;
+    pub mod escape;
+    pub mod format;
     pub mod layout;
     pub mod markdown;
-    pub mod format;
+    pub mod render;
 }
 pub mod passes;
 pub mod plugins;