Fixed some problems with PersistentList

Persistent list is/was buggy. I think it still is, I'm not exactly sure what kind of design you wanted to go for, but I did a small fix that helped to solve my problems and the unit tests seem to still run.

For example, a simple scenario that failed: `person.children.contains(person.children[0])`. There were more, like `.forEach()` and `.first`, etc. not working properly.

Please figure out these bugs.