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
For macros that you normally use at the class level to define instance methods, but instead want to use to define class methods, you have to the singleton class as you have here, so that's fine.  The preference I mentioned before is for normal static method definitions being done using `def self.method_name`, but for things that really need to run in the context of the singleton class, this is fine.