http://www.robohornet.org gives a score of 101.36 on master,
and 102.68 with this PR. The latter is slightly better,
but probably within noise level.
So it looks like this PR does not affect DOM performance.
This is expected since `Box::new` is defined as:
```rust
impl<T> Box<T> {
#[inline(always)]
pub fn new(x: T) -> Box<T> {
box x
}
}
```
With inlining, it should compile to the same as box syntax.
In a later PR, DomRoot<T> will become a type alias of Root<Dom<T>>,
where Root<T> will be able to handle all the things that need to be
rooted that have a stable traceable address that doesn't move for the
whole lifetime of the root. Stay tuned.
Check if represented attributes are cached
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Improve existing cache checks in `/bluetooth/lib.rs`, and update Step comments in `script/dom` corresponding to this.
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It could be used to have mutable JSVal fields without GC barriers.
With the removal of that trait, MutHeap and MutNullableHeap can respectively
be replaced by MutJS and MutNullableJS.
Move the AttributeInstanceMaps from bluetooth to bluetoothDevice.
<!-- Please describe your changes on the following line: -->
The previous implementation differed from the spec, because there was three maps instead of one. With this, they will be merged into one.
Also this map has been moved from bluetooth to bluetoothDevice, because its make more sense to store it there. There is an issue about it [here](https://github.com/WebBluetoothCG/web-bluetooth/issues/330).
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