Deprecate debugger

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 969768cbd04a5662b7e1a5f11fc5ab4fe8267cd5.  I'm curious what the rest of you think...is this the right approach?

/cc @alindeman @JonRowe @samphippen @soulcutter