Auto merge of #13641 - emilio:rayon-style, r=pcwalton

Use rayon to drive parallel layout and styling.

<!-- Please describe your changes on the following line: -->

The current work queue had a really annoying constraint: The size of the node had to be the size of the work unit data.

This makes it impractical for the new restyling model where we plan to pass down a bunch of data.

Rayon by default makes you wait for the result of the work unit, which makes it impractical for the current model (it's mostly sequential).

I added an API to rayon that allows us to push work to the queue without waiting (https://github.com/nikomatsakis/rayon/pull/103).

This still needs some work (for example, we're loosing the memory reporting functionality), but I wanted feedback on this.

---

<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->

---

This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/13641)

<!-- Reviewable:end -->

0 files changed, 0 insertions, 0 deletions