Box Typedef to reduce the size of ItemKind

This reduces the size from

```
[src/librustdoc/lib.rs:102] std::mem::size_of::<Item>() = 520
[src/librustdoc/lib.rs:102] std::mem::size_of::<ItemKind>() = 248
```

to

```
[src/librustdoc/lib.rs:102] std::mem::size_of::<Item>() = 432
[src/librustdoc/lib.rs:102] std::mem::size_of::<ItemKind>() = 160
```

Author: jyn514

src/librustdoc/clean/inline.rs

@@ -77,7 +77,7 @@ crate fn try_inline(
         Res::Def(DefKind::TyAlias, did) => {
             record_extern_fqn(cx, did, clean::TypeKind::Typedef);
             ret.extend(build_impls(cx, Some(parent_module), did, attrs));
-            clean::TypedefItem(build_type_alias(cx, did), false)
+            clean::TypedefItem(box build_type_alias(cx, did), false)
         }
         Res::Def(DefKind::Enum, did) => {
             record_extern_fqn(cx, did, clean::TypeKind::Enum);

src/librustdoc/clean/mod.rs

@@ -1121,7 +1121,7 @@ impl Clean<Item> for hir::ImplItem<'_> {
                 hir::ImplItemKind::TyAlias(ref ty) => {
                     let type_ = ty.clean(cx);
                     let item_type = type_.def_id().and_then(|did| inline::build_ty(cx, did));
-                    TypedefItem(Typedef { type_, generics: Generics::default(), item_type }, true)
+                    TypedefItem(box Typedef { type_, generics: Generics::default(), item_type }, true)
                 }
             };
             Item::from_def_id_and_parts(local_did, Some(self.ident.name.clean(cx)), inner, cx)
@@ -1270,7 +1270,7 @@ impl Clean<Item> for ty::AssocItem {
                     let type_ = cx.tcx.type_of(self.def_id).clean(cx);
                     let item_type = type_.def_id().and_then(|did| inline::build_ty(cx, did));
                     TypedefItem(
-                        Typedef {
+                        box Typedef {
                             type_,
                             generics: Generics { params: Vec::new(), where_predicates: Vec::new() },
                             item_type,
@@ -1981,7 +1981,7 @@ impl Clean<Vec<Item>> for (&hir::Item<'_>, Option<Symbol>) {
                     let rustdoc_ty = ty.clean(cx);
                     let item_type = rustdoc_ty.def_id().and_then(|did| inline::build_ty(cx, did));
                     TypedefItem(
-                        Typedef { type_: rustdoc_ty, generics: generics.clean(cx), item_type },
+                        box Typedef { type_: rustdoc_ty, generics: generics.clean(cx), item_type },
                         false,
                     )
                 }

src/librustdoc/clean/types.rs

@@ -307,7 +307,7 @@ crate enum ItemKind {
     EnumItem(Enum),
     FunctionItem(Box<Function>),
     ModuleItem(Module),
-    TypedefItem(Typedef, bool /* is associated type */),
+    TypedefItem(Box<Typedef>, bool /* is associated type */),
     OpaqueTyItem(OpaqueTy),
     StaticItem(Static),
     ConstantItem(Constant),

src/librustdoc/html/render/mod.rs

@@ -3561,7 +3561,7 @@ fn render_deref_methods(
         .items
         .iter()
         .find_map(|item| match item.kind {
-            clean::TypedefItem(ref t, true) => Some(match *t {
+            clean::TypedefItem(ref t, true) => Some(match **t {
                 clean::Typedef { item_type: Some(ref type_), .. } => (type_, &t.type_),
                 _ => (&t.type_, &t.type_),
             }),
@@ -4239,7 +4239,7 @@ fn sidebar_assoc_items(it: &clean::Item) -> String {
             {
                 if let Some((target, real_target)) =
                     impl_.inner_impl().items.iter().find_map(|item| match item.kind {
-                        clean::TypedefItem(ref t, true) => Some(match *t {
+                        clean::TypedefItem(ref t, true) => Some(match **t {
                             clean::Typedef { item_type: Some(ref type_), .. } => (type_, &t.type_),
                             _ => (&t.type_, &t.type_),
                         }),

src/librustdoc/json/conversions.rs

@@ -179,7 +179,7 @@ impl From<clean::ItemKind> for ItemEnum {
             StaticItem(s) => ItemEnum::StaticItem(s.into()),
             ForeignStaticItem(s) => ItemEnum::StaticItem(s.into()),
             ForeignTypeItem => ItemEnum::ForeignTypeItem,
-            TypedefItem(t, _) => ItemEnum::TypedefItem(t.into()),
+            TypedefItem(box t, _) => ItemEnum::TypedefItem(t.into()),
             OpaqueTyItem(t) => ItemEnum::OpaqueTyItem(t.into()),
             ConstantItem(c) => ItemEnum::ConstantItem(c.into()),
             MacroItem(m) => ItemEnum::MacroItem(m.source),