layout: Clean up inline layout data types a bit (#34563)

- Remove the `LayoutBox::InlineBox` variant that was only used for
  inline level boxes. Now they are stored in `LayoutBox::InlineLevel`
  along with other kinds of out-of-flow and atomic inline items.
- Reduce the size of `InlineItem` by 260 bytes per item by using atomic
  indirection / pointers. This adds a bit of overhead to access items in
  exchange for a lot of memory saved.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Oriol Brufau <obrufau@igalia.com>

components/layout_2020/flow/root.rs

@@ -185,7 +185,6 @@ impl BoxTree {
                         },
                         _ => return None,
                     },
-                    LayoutBox::InlineBox(_) => return None,
                     LayoutBox::InlineLevel(inline_level_box) => match &*inline_level_box.borrow() {
                         InlineItem::OutOfFlowAbsolutelyPositionedBox(_, text_offset_index)
                             if box_style.position.is_absolutely_positioned() =>