Response API
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This PR adds the [dom::Response](https://fetch.spec.whatwg.org/#response-class) implementation and addresses #11896.
The relevant passing tests` expectations have been updated.
In order to allow non-UTF-8-encoded status messages, `net_traits::response::Response`'s `raw_status` field has been changed from type [`Option<RawStatus>`](https://doc.servo.org/hyper/http/struct.RawStatus.html) to type `Option<(u16, Vec<u8>)>`. As a result, a few other files which rely on the `raw_status` field were affected and updated.
TODOs:
- The `body` and `trailer` methods. Relies on implementation of `ReadableStream` and `Promise`s.
- Similarly, replace the dummy constructor `_body: Option<USVString>` argument with `body: ResponseBodyInit`.
- Currently, whenever `r's response's header list` or `r's Headers object` are mentioned, I always modify the `headers_reflector` field (of type dom::Headers, or `r's Headers object`) and not the corresponding hyper::Headers list in the net_traits::Response field. A completely accurate interpretation of the spec might consider making both of these lists the same thing via a reference. [Discussion](https://github.com/whatwg/fetch/issues/358) was [had](https://github.com/servo/servo/pull/12884).
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Add lots of Arc’s in style, and prepare for using DOMRefCell
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`DOMRefCell` usage is not there year because of thread-safety questions, but I have this much already that I’d like to land before it bitrots.
r? @emilio
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Convert imports in generate_imports into one per line, so that it can…
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Convert imports in generate_imports into one per line, so that it can easily add new ones with alphabetical order.
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… easily add new ones with alphabetical order.
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Fix ridiculous DOM proxy getter performance
This implements the missing shadowing checks that were causing us to take many slow paths when dealing with proxy objects. Verified by running `tests/html/binding_perf.html` before and after and observing a 12x improvement.
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Support pair and value iterable WebIDL bindings
The actual iterator implementation and JSAPI calls related to setting up the interface are ported directly from Gecko's Codegen.py, IterableIterator.h, and IterableIterator.webidl. The changes to support multiple interfaces in one file are required because the internal iterator interface the parser generates gets associated with the original interface's WebIDL file. It seemed like a good time to address #571 in that case.
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