rustdoc: Represent item types as a small number in the search index.

Has negligible improvements with gzip, but saves about 7% without it.
This also has an effect of changing the tie-breaking order of item types.

src/librustdoc/lib.rs

@@ -41,6 +41,7 @@ pub mod fold;
 pub mod html {
     pub mod highlight;
     pub mod escape;
+    pub mod item_type;
     pub mod format;
     pub mod layout;
     pub mod markdown;