Rock accepts assignment between generics with different type

[horasal]

interesting:

data: ArrayList<ArrayList<Int>>
data = ArrayList<Int> new()
data add(ArrayList<Int> new)
data[0] add(1)

This can be compiled with rock, and sometimes works. But this code is illegal. Because libgc will collect elements at anytime. So we will get a random segmentation fault.
We should at least throw a warning on this.