Memory leaks on process shutdown

Our ref counting is indicating memory leaks on process shut down.

It looks like this is probably due to a circular reference between App Domains and the Module Infos contained therein. That circular reference is meant to be broken when we get a module unload callback, but that callback is not guaranteed on process shut down. We will likely also leak App Domain references on shut down because I don't think we are guaranteed to get a callback for that either. Essentially, I think all bets are off on callbacks during process exit.

We might need to have better memory leak detection. It probably wouldn't hurt to be a little better about circular references anyway. For example, there is no need to hold a ref-counted reference to the App Domain from the Module Info, since the App Domain should keep it alive.

Start by tracing the App Domain and Module Info reference relationships and the module-unload and process-shutdown callback paths described in the issue. Done means shutdown no longer reports the identified leaks, circular references are handled safely, and leak detection covers exit paths where callbacks are absent.