Barrier insertion updates

• Dependencies are now tracked based on a list of active potential sources, and the active set is narrowed by filtering through readers/writers
• Stricter handling of dependencies within loops by not assuming that loops are nonempty
• Changes the barrier insertion algorithm to act in two stages: first, barriers are recursively inserted into loops, then they are inserted at the outermost level. This is mostly meant to simplify the code.
• See also #18 on gitlab