Support break statements

[j-hui]

The code and syntax for this is actually already there ("completed" in 152d994), but I took it out in 669814e.

The problem is break completely breaks (ROFL) reference counting, because break ing out of a loop forgoes any inserted drops. The fix might be to rewrite how codegen handles loops, or to rewrite how codegen handles drops. We may even need to rewrite most of codegen.