SI-8153 Mutation is hard, let's go shopping with an empty list

Changed the implementation of iterator to be more robust to mutation of the underlying ListBuffer.

Added a test to make sure bug is gone.
Fixed an "unsafe" usage of ListBuffer.iterator in the compiler, and added a comment explaining the (new) policy for iterating over a changing ListBuffer.

The compiler now uses ArrayBuffer instead of ListBuffer, as that makes the (custom) units Iterator more natural to write (and avoids O(n) lookups).