Auto merge of #12571 - servo:selectors-generic-atom_, r=bholley

Update to selectors 0.8.2 (generic over atoms)

This removes the `[replace]` override in geckolib and therefore unblocks https://github.com/servo/servo/pull/12391.

This includes the `gecko_string_cache` redesign discussed in https://github.com/servo/servo/pull/12548.

r? @bholley

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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 fix #12548 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [x] These changes do not require new tests because refactor

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tests/unit/style/Cargo.toml

@@ -14,7 +14,7 @@ app_units = "0.2.5"
 cssparser = {version = "0.5.4", features = ["heap_size"]}
 euclid = "0.7.1"
 rustc-serialize = "0.3"
-selectors = {version = "0.7", features = ["heap_size"]}
+selectors = {version = "0.8", features = ["heap_size"]}
 string_cache = {version = "0.2", features = ["heap_size"]}
 style = {path = "../../../components/style"}
 style_traits = {path = "../../../components/style_traits"}