Pass `&mut JSContext` to hooks

[TimvdLippe]

All hooks currently pass in cx: *mut RawJSContext,. In Servo, we have these blocks in a lot of the hooks:

let mut cx = unsafe {
    // SAFETY: We are in SM hook
    JSContext::from_ptr(NonNull::new(cx).expect("JSContext should not be null in SM hook"))
};
let cx = &mut cx;

Instead of passing the raw jscontext to the hook, it would be nicer to pass in the &mut JSContext to avoid all of these conversions in Servo. That would be a breaking change though, but I think it's manageable.

[sagudev]

This get's tricky because the hooks are called from SM/C++ directly and sometimes they are stored in the structs. I think we have similar pattern in servo where we light wrapper for full rusty hook. I found only macro: https://github.com/servo/servo/blob/77c9f634c59c9f4f6a2e9c734cfd3ad6828cd6c4/components/script/dom/bindings/function.rs#L28, but there should be one done with generics or closures too.

So rather than changing existing methods we should just offer this hook wrappers in mozjs (could we autogen them?).