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 reasons I don't fully understand, using `yield` is about 5x faster than `&block` and `block.call`.  I'd prefer to see this method not capture the block with `&block` and instead use `yield`.  Here's a benchmark, FWIW:

https://github.com/rspec/rspec-core/blob/0c4f4fbcdc04aac6e27efad934bd40d3eaaf0b6c/benchmarks/capture_block_vs_yield.rb