Closures capturing scripts should not (ab)use properties to keep the reference

A closure needs to make sure that it keeps the script array of its context alive (if it references variables from there). Currently, we use an invalid property to tell the gc about it; however, an alternative would be to have dedicated Function subclasses to track them.

We don't want to add a member to reference scrips to every function though, as that functionality is only needed when scripts are actually imported, which isn't that common.