Add debug gem and mark slow specs

[vburzynski]

• adds the debug gem
  • to make it easier to set breakpoints via an invocation of debugger
• marks the slowest specs with :slow
  • use bundle exec rspec --tag '~slow' to skip slow tests
  • use bundle exec rspec --profile to see slowest tests
  • this makes it possible to run the bulk of the test suite in roughly 1 second as opposed to roughly 5 seconds
  • namely ./spec/acceptance/sequence_setting_spec.rb:309 alone was taking up 61% of the time to run the entire suite (3.12 seconds out of the 5.09 seconds). It seems like a useful test to retain, but is not critical to run while developing other features.

[vburzynski]

😅 ah jruby and truffleruby don't like the addition of debug

• Make the debug gem work on TruffleRuby ruby/debug#257
• JRuby support ruby/debug#1120

[vburzynski]

ok, even though conditionally using add_development_dependency works in the gemspec... It feels off. Maybe it's better to keep the bring-your-own-debugger approach for now rather than add the convenience of putting one in as a dev dependency.

I'll open a new PR for just marking :slow specs