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
I know you don't usually open up the singleton class, but I was getting a `private method called` error when trying to use `self.attr_writer` - any idea?