Auto merge of #11816 - servo:stable-style, r=nox

Make the style crate (almost) build with a stable compiler

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The bulk of this is adding cargo features to make derived implementations of `heapsize` and `serde` traits optional.

"Almost" because `std::intrinsics::discriminant_value` is currently unstable and doesn’t have any stable replacement that I know of. For now, this PR conditionally replaces it with `unimplemented!()`.

r? @nox

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes <s>fix</s> are part of #11815 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [x] These changes do not <s>require tests</s> *have tests yet* because that requires https://github.com/servo/servo/issues/11806, but I still want to land this before it bitrots. (Tests should check that the build succeeds with a stable compiler.)

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