Show stacks for unpoison & poison

Originally reported on Google Code with ID 191

```
For normal malloc/free memory, a use-after-free results in the stack for the allocation,
the deallocation, and the final use.

For arena memory, a use-after-poison would ideally have the stack for the most recent
unpoison, the poison, and the final use. In addition to the allocation for the arena
itself, I guess.
```

Reported by `jruderman` on 2013-06-04 19:55:33

The issue names no source files, tests, or entry points. Start by locating the arena poison and unpoison reporting paths and their existing stack-capture behavior; done means a use-after-poison report includes the most recent unpoison, poison, final-use, and arena-allocation stacks.

Written by the indexing model from the issue text.