Auto merge of #8286 - eefriedman:no-move, r=nox

Remove unnecessary uses of #[no_move]

The patch makes RootCollection a bit safer by making the StackRootTLS hold
it in place.

RootedVec was doing an extremely delicate dance and just hoping nobody
messed it up; switch to a Box to be safe.

CodeGenRust seemed to be using no_move for no particularly good reason.

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components/script/dom/bindings/js.rs

@@ -454,7 +454,6 @@ impl<T: Reflectable> OptionalRootedReference<T> for Option<Option<Root<T>>> {
 ///
 /// See also [*Exact Stack Rooting - Storing a GCPointer on the CStack*]
 /// (https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey/Internals/GC/Exact_Stack_Rooting).
-#[no_move]
 pub struct RootCollection {
     roots: UnsafeCell<Vec<*const Reflector>>,
 }