rustdoc: sort deprecated items lower in search

serialize `q` (`itemPaths`) sparsely
overall 4% reduction in search index size

src/librustdoc/html/render/mod.rs

@@ -107,6 +107,7 @@ pub(crate) struct IndexItem {
     pub(crate) parent_idx: Option<usize>,
     pub(crate) search_type: Option<IndexItemFunctionType>,
     pub(crate) aliases: Box<[Symbol]>,
+    pub(crate) deprecation: Option<Deprecation>,
 }
 
 /// A type used for the search index.