Per-compartment snapshotting

[asajeffrey]

It would be nice if we could snapshot some compartments, not others. For example when deep-cloning cloning an object from one compartment to another, it would be nice if we could rely on the source compartment being snapshotted, so we could copy from it safely.

At the moment the code to clone an array of objects is https://github.com/asajeffrey/linjs/blob/1bcfc4bfdfd975d00c954c0be29de9172e34c3a2/examples/minidom/minidom.rs#L181-L196

This is tricker than it should be to cover the case where C and D are the same compartment, so updates to the target might change the state of the source, which is why we can't just iterate over the source array. If D was snapshotted but C wasn't then we could write simpler code, something like:

fn clone_children_from<S, D>(self, cx: &'a mut JSContext<S>, element: Element<'a, D>) where
    S: CanAccess + CanAlloc + InCompartment<C> + Snapshotted<'a, D>,
    C: Compartment,
    D: Compartment,
{
    let children = element.0.borrow(cx).children.snapshot(cx);
    for &child in &children {
        self.append_clone(cx, child);
    }
}