Auto merge of #11835 - servo:diy-as-unsafe-cell, r=nox

Use our copy of RefCell for style data

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

This allows removing `#![feature(as_unsafe_cell)]` in geckolib and make progress towards #11815.

---
<!-- 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 part of #11815.

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- [ ] There are tests for these changes OR
- [x] These changes do not require new tests because they don’t introduce no functional changes.

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ports/geckolib/lib.rs

@@ -2,8 +2,6 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-#![feature(as_unsafe_cell)]
-
 extern crate app_units;
 #[macro_use]
 extern crate cssparser;

ports/geckolib/wrapper.rs

@@ -28,7 +28,6 @@ use selectors::Element;
 use selectors::matching::DeclarationBlock;
 use selectors::parser::{AttrSelector, NamespaceConstraint};
 use smallvec::VecLike;
-use std::cell::{Ref, RefCell, RefMut};
 use std::marker::PhantomData;
 use std::ops::BitOr;
 use std::ptr;
@@ -46,6 +45,7 @@ use style::parser::ParserContextExtraData;
 #[allow(unused_imports)] // Used in commented-out code.
 use style::properties::parse_style_attribute;
 use style::properties::{PropertyDeclaration, PropertyDeclarationBlock};
+use style::refcell::{Ref, RefCell, RefMut};
 use style::restyle_hints::ElementSnapshot;
 use style::selector_impl::ElementExt;
 #[allow(unused_imports)] // Used in commented-out code.