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
We use YARD for docs.  You can run a YARD server locally by running `bundle exec yard server --reload` -- then you can access the docs at localhost:8808 to see how they render.  If you want examples of how we doc things with YARD, these files contain a lot of YARD doc comments:

https://github.com/rspec/rspec-core/blob/master/lib/rspec/core/configuration.rb
https://github.com/rspec/rspec-core/blob/master/lib/rspec/core/example_group.rb