Auto merge of #20399 - Xanewok:remove-heap-handle-mut, r=jdm

Sanitize Heap::handle(_mut) functions

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Complementary to https://github.com/servo/rust-mozjs/pull/404.

Removing `Heap::handle_mut` didn't warrant any changes on Servo side, and so the changes here are only to fix compilation with `Heap::handle` being now marked as `unsafe`.

The main idea is that we can't hand out handles to heap values themselves, since they're not guaranteed to be rooted, but it's safe to do when we are - hence why the safe impl on `RootedTraceableBox<Heap<T>>` and why it's safe to use inside structs that hold a Heap and are `#[must_root]`.

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