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 only opened the singleton class because I had a bunch of methods I was writing, but with your previous comments I have gotten rid of them.  Switched back to `def self.class_method` style.

How do I go about making docs?  Just RDoc in the comments?