servo: Merge #17565 - Servo's async Tokenizer: New! Improved! Now wit…

…h multithreading! (from cynicaldevil:new-parser-thread); r=nox,jdm

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The Tokenizer, defined in the file `async_html.rs` will run on the main thread, while h5e's `Tokenizer`(along with its Sink) lives on the parser thread. Both h5e's `Tokenizer` and `Sink` communicate with the main thread Tokenizer via their own mpsc channels. The Sink keeps sending parser operations to the main thread, to be executed. For some operations, it waits for a message from the main thread before returning.

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Source-Repo: https://github.com/servo/servo
Source-Revision: e8272dcf1a316549cdbc98a16c181c02743041c4