Use Tera templates for rustdoc.

Replaces a format!() call in layout::render with a template
expansion. Introduces a `templates` field in SharedContext so parts
of rustdoc can share pre-rendered templates.

This currently builds in a copy of the single template available, like
with static files. However, future work can make this live-loadable with
a perma-unstable flag, to make rustdoc developers' work easier.

src/librustdoc/html/render/write_shared.rs

@@ -460,7 +460,14 @@ pub(super) fn write_shared(
                     })
                     .collect::<String>()
             );
-            let v = layout::render(&cx.shared.layout, &page, "", content, &cx.shared.style_files);
+            let v = layout::render(
+                &cx.shared.templates,
+                &cx.shared.layout,
+                &page,
+                "",
+                content,
+                &cx.shared.style_files,
+            );
             cx.shared.fs.write(&dst, v.as_bytes())?;
         }
     }