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](https://github.com/servo/rust-mozjs/blob/ed5e37a288b5738d9b571b8100b4a22a2c00f075/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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