rustdoc: tweak the search index format

This essentially switches search-index.js from a "array of struct"
to a "struct of array" format, like this:

    {
    "doc": "Crate documentation",
    "t": [ 1, 1, 2, 3, ... ],
    "n": [ "Something", "SomethingElse", "whatever", "do_stuff", ... ],
    "q": [ "a::b", "", "", "", ... ],
    "d": [ "A Struct That Does Something", "Another Struct", "a function", "another function", ... ],
    "i": [ 0, 0, 1, 1, ... ],
    "f": [ null, null, [], [], ... ],
    "p": ...,
    "a": ...
    }

So `{ty: 1, name: "Something", path: "a::b", desc: "A Struct That Does Something", parent_idx: 0, search_type: null}` is the first item.

This makes the uncompressed version smaller, but it really shows on the
compressed version:

    notriddle:rust$ wc -c new-search-index1.52.0.js
    2622427 new-search-index1.52.0.js
    notriddle:rust$ wc -c old-search-index1.52.0.js
    2725046 old-search-index1.52.0.js
    notriddle:rust$ gzip new-search-index1.52.0.js
    notriddle:rust$ gzip old-search-index1.52.0.js
    notriddle:rust$ wc -c new-search-index1.52.0.js.gz
    239385 new-search-index1.52.0.js.gz
    notriddle:rust$ wc -c old-search-index1.52.0.js.gz
    296328 old-search-index1.52.0.js.gz
    notriddle:rust$

That's a 4% improvement on the uncompressed version (fewer `[]`),
and 20% improvement after gzipping it, thanks to putting like-typed
data next to each other. Any compression algorithm based on a sliding
window will probably show this kind of improvement.

src/librustdoc/html/render/mod.rs

@@ -166,23 +166,6 @@ crate struct IndexItem {
     crate search_type: Option<IndexItemFunctionType>,
 }
 
-impl Serialize for IndexItem {
-    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
-    where
-        S: Serializer,
-    {
-        assert_eq!(
-            self.parent.is_some(),
-            self.parent_idx.is_some(),
-            "`{}` is missing idx",
-            self.name
-        );
-
-        (self.ty, &self.name, &self.path, &self.desc, self.parent_idx, &self.search_type)
-            .serialize(serializer)
-    }
-}
-
 /// A type used for the search index.
 #[derive(Debug)]
 crate struct RenderType {