Attr is a Node, with consequences for many Node methods
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Attr is now a Node, as current WHATWG specs require. I think I did some unidiomatic things to make compareDocumentPosition work and it could use a look by someone more familiar with how to write concise Rust code. I also think the new cases in compareDocumentPosition lack tests; it is possible to compare the position of two attributes, or of an attribute and an element, and I don't think any tests are exercising that functionality.
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix#25124
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- [ ] There are tests for these changes (existing cases of Node methods are well-tested, and there is a WPT test specifically for whether Attr is Node, but I'm not sure about new Node method cases)
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There are a few canvas2d-related dependencies that haven't updated, but they
only use euclid internally so that's not blocking landing the rest of the
changes.
Given the size of this patch, I think it's useful to get this landed as-is.
registered_mutation_observers() returns immutable references and does
not init mutation observers.
registered_mutation_observers_mut() lazily initializes raredata if it
does not exist.
Updated code that uses this methods to call appropriate method when
mutation is not necessary.
