Deprecation formatter

[dchelimsky]

Per convo in #912, print deprecations through a new DeprecationFormatter, and change the deprecation API to:

RSpec.deprecate("thing to be deprecated", :replacement => "it's replacement")
RSpec.warn_deprecation("custom message that is arguably better than the message generated by RSpec.deprecate for a specific scenario")

[fables-tales]

Nice job David! This looks great.

[dchelimsky]

FYI - I added a last minute addition to change a hash key (internal). This is ready to go AFAIC.

[myronmarston]

I think the noise of combining what is usually on multiple lines (via semicolons) inhibits clarity here. What do you think about using a more normal code layout like this?

class Foo
  def bar
    RSpec.deprecate "Foo#bar"
  end
end

Alternately, if you don't want all 5 lines, you could just define it as a top-level method directly on main, and then call it from the example below. That'd cut it down to 3 lines.

[dchelimsky]

Fixed in e8c1c59

[myronmarston]

👍 Fantastic work, David. I left a few questions, but I'm completely happy with it as is. I'll hold off merging for now in case there's any followup from my questions.