Deprecate debugger

Created by: myronmarston

BTW, as I mentioned in rspec/rspec-expectations#286, I'd like for us to be more intentional about making sure the call sites in deprecations are reported properly. To that end, we need to spec it. I realized that it's not particularly easy to do so, so I added some helper methods in 969768cb. I'm curious what the rest of you think...is this the right approach?

/cc @alindeman @JonRowe @samphippen @soulcutter