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
Yeah, I think I'd prefer we don't do the block checking here.  Ruby's not a statically-typed language and IMO doing these sorts of checks goes against the philosophy of how ruby is designed to be used.

Also, the simple act of capturing `&block`, even if you `yield` instead, has a perf cost:

https://github.com/rspec/rspec-core/blob/master/benchmarks/capture_block_vs_yield.rb

...so I'd like to see `&block` removed entirely.