Refactoring Sandboxing to be in RSpec support

rspec-core changes for https://github.com/rspec/rspec-core/issues/1798

rspec-support changes in https://github.com/rspec/rspec-support/pull/137
Two things:

* I don't think the new object that extends `RSpec::Core::SharedExampleGroup` is necessary.  I looked through the history and that dates back to the initial creation of the sandbox in 92e0fc44f9ff4c3b7988851f9701d4a3c72e70d5. I'm not sure why it was needed then, but I tried removing it and the specs all pass (sans to formatting specs that had backtrace lines change, but which are easy to fix).  So this can just be `RSpec::Mocks.with_temporary_scope(&block)`.
* I don't think it makes sense for rspec-core's sandboxing to automatically apply a temporary rspec-mocks scope.  Users that want both sandboxes can easily compose them:

``` ruby
RSpec.configure do |c|
  c.around do |ex|
    RSpec::Core::Sandboxing.sandboxed do
      RSpec::Mocks.with_temporary_scope { ex.run }
    end
  end
end
```

...and in fact, that's what we'd do in rspec-core's spec sandbox.