Eliminate a mem::transmute in CodeGen
<!-- Please describe your changes on the following line: -->
Eliminate a mem::transmute in CodeGen by changing the find_enum_string_index function to take a slice of pairs and return an enum value.
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix#15587 (github issue number if applicable).
<!-- Either: -->
- [ ] There are tests for these changes OR
- [X] 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/15659)
<!-- Reviewable:end -->
It is only used once, in a case where it can never have an effect: in case
for it to do something, both returnType.isDictionary() and
returnType.isGeckoInterface() would need to be true at the same time.
Remove usage of phf_macros.
It’s a compiler plugin that uses unstable compiler APIs that are not on a path to stabilization.
With this changes, there is one less thing that might break when we update the compiler. For example: https://github.com/sfackler/rust-phf/pull/101
<!-- Please describe your changes on the following line: -->
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./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/15099)
<!-- Reviewable:end -->
Make WebIDL callbacks permanently rooted
This replicates the same model that Promise uses right now, because it requires less thinking than coming up with something else.
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix#14447
- [ ] There are tests for these changes
<!-- 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/14994)
<!-- Reviewable:end -->
