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.
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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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