Fix unstubing existing instances of any instance
* access proxies for any instance of klass
* remove stubs from "running" instances
* pass recorder into chains to register stubs
* reduce lookup cost of proxies by klass
* record stubs used
* remove just the stubs recorded by any instance
* switch from for..in to each
* allow unstubing of instances that are a sub class
* refactor lookup for subclass
* Improve doc string.
* "local instance" didn't really give me the right
* sense for what this was testing.
* Improve space spec.
    - The old spec only checked the number of proxies returned,
      and didn't actually check that it returned the right ones.
    - The old spec only tested who were instances of the given
      class, and did not check instances of subclasses.
Merge pull request #402 from rspec/prevent_errors_on_stub_removal

Prevent errors on stub removal