rustdoc: sort search index items for compression

This should not affect the appearance of the docs pages themselves.

This makes the pre-compressed search index smaller, thanks to the
empty-string path duplication format, and also the gzipped version,
by giving the algorithm more structure to work with.

    rust$ wc -c search-index-old.js search-index-new.js
    2628334 search-index-old.js
    2586181 search-index-new.js
    5214515 total
    rust$ gzip search-index-*
    rust$ wc -c search-index-old.js.gz search-index-new.js.gz
    239486 search-index-old.js.gz
    237386 search-index-new.js.gz
    476872 total

src/librustdoc/html/render/mod.rs

@@ -164,6 +164,7 @@ crate struct IndexItem {
     crate parent: Option<DefId>,
     crate parent_idx: Option<usize>,
     crate search_type: Option<IndexItemFunctionType>,
+    crate aliases: Box<[String]>,
 }
 
 /// A type used for the search index.