auto merge of #5176 : brson/rust/unwrap_shared_mutable_state, r=nikomatsakis

auto merge of #5176 : brson/rust/unwrap_shared_mutable_state, r=nikomatsakis

r?

This fixes the current [random failures](http://buildbot.rust-lang.org/builders/auto-linux/builds/291/steps/test/logs/stdio) 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 and is not used outside of test cases.

Note that there is not consensus to remove it.

(second commit)