Use Heap instead of UnsafeCell in DOM reflectors
The previous `Reflector` implementation did not use post barriers, so we could crash when storing nursery objects in a `Reflector` structure that were later moved out of the nursery.
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix#15085
- [X] There are tests for these changes
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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
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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
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It could be used to have mutable JSVal fields without GC barriers.
With the removal of that trait, MutHeap and MutNullableHeap can respectively
be replaced by MutJS and MutNullableJS.
