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
Now that we are exposing the sandboxing as a public API, I think we can add these writers to the definition of `RSpec` in `lib/rspec/core.rb.` (Just be sure to label them `@api private`).  We had them in the spec stuff because they weren't needed by any public APIs but now that they are they belong in `lib/rspec/core.rb`, I think.