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 prefer the `def self.class_method` style over opening the singleton class like this.

Also, now that we're making this a public API, I'm thinking that maybe `RSpec::Core::Sandbox` works better as the name as that's a noun?

It'll need docs too, eventually.