Unwrap shared mutable state

r?

This fixes the current random failures on the bots and closes #4436 by removing `unwrap_shared_mutable_state` and the code that depends on it. The result is that ARC-like things will not be unwrappable. This feature is complex, has no current use cases, and I don't want to maintain it. Note that removing it is against @bblum's objections.