Box ItemKind to reduce the size of Item

This brings the size of `Item` from

```
[src/librustdoc/lib.rs:103] std::mem::size_of::<Item>() = 680
```

to

```
[src/librustdoc/lib.rs:103] std::mem::size_of::<Item>() = 280
```

src/librustdoc/passes/calculate_doc_coverage.rs

@@ -187,7 +187,7 @@ impl<'a, 'b> CoverageCalculator<'a, 'b> {
 
 impl<'a, 'b> fold::DocFolder for CoverageCalculator<'a, 'b> {
     fn fold_item(&mut self, i: clean::Item) -> Option<clean::Item> {
-        match i.kind {
+        match *i.kind {
             _ if !i.def_id.is_local() => {
                 // non-local items are skipped because they can be out of the users control,
                 // especially in the case of trait impls, which rustdoc eagerly inlines