Auto merge of #19644 - Xanewok:root-seq-any, r=jdm

Root sequence<any> params using CustomAutoRooter

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Attempt at https://github.com/servo/servo/issues/16678. At the moment this pulls an external [remove-handle-conv](https://github.com/Xanewok/rust-mozjs/tree/remove-handle-conv) branch for rust-mozjs (removing `FromJSValConvertible` implementation for `HandleValue` and adding one for `*mut JSObject`)

In short, this roots the `sequence<any>` and `sequence<object>` function arguments using `CustomAutoRooter` (introduced in https://github.com/servo/rust-mozjs/pull/382).

As per https://github.com/servo/servo/issues/16678#issuecomment-302224110 the underlying vector contains raw gc thing pointers instead of handles. To do that, this introduces new possible `isMember = "AutoRoot"` value in CodegenRust.py.
The rooted argument is passed to a function wrapped in a `CustomAutoRooterGuard` [RAII root guard](ed5e37a288/src/rust.rs (L586-L588)) (e.g. as `CustomAutoRooterGuard<Vec<JSVal>>`)

I felt quite out of my depth when trying to adapt CodegenRust.py code, so I'd appreciate any help with how can be done better/in a more clean manner.
Also the name `CustomAutoRooterGuard` is quite lengthy, so I'm also open to changing it (`AutoRoot`? `AutoRootGuard`?)

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- [x] These changes fix #16678  (github issue number if applicable).

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components/script/dom/webidls/TestBinding.webidl

@@ -263,6 +263,9 @@ interface TestBinding {
   void passCallbackFunction(Function fun);
   void passCallbackInterface(EventListener listener);
   void passSequence(sequence<long> seq);
+  void passAnySequence(sequence<any> seq);
+  sequence<any> anySequencePassthrough(sequence<any> seq);
+  void passObjectSequence(sequence<object> seq);
   void passStringSequence(sequence<DOMString> seq);
   void passInterfaceSequence(sequence<Blob> seq);