Memoize exception_lines in ExceptionPresenter

[MaxLap]

When an exception occurs and is displayed, ExceptionPresenter is used. However, there is an obvious slow amplification doing in it. This PR fixes it.

The method failure_lines uses exception_lines 3 times, which uses exception.message. In some situation, exception.message can be slow (when inspect is called on a big object for example, such as during NoMethodError).
Without caching, this delay, which can become considerable, will be amplified by 3x.

Here is a simple example of an rspec that would be amplified

class A
  def inspect
    sleep(1)
  end
end

describe 'A' do
  it 'works' do
    A.new.no_such_method
  end
end

Instead of taking 1 second, the print of the failure message takes 3 seconds.

I did memoization (@exception_lines ||= ...), but the call result could have been saved and reused in the failure_lines method. Let me know if you prefer that i change it to just start failure_lines with exception_lines = self.exception_lines.

As a side note, I think it would be helpful if the --profile or a new flag would help debug slow failures messages (top 10 of slowest failures to display).

[pirj]

Do you mind checking the build failure?

[MaxLap]

Looking here https://travis-ci.org/github/rspec/rspec-core/branches, i believe the problem is on master, and not a result of my changes.

[JonRowe]

@MaxLap It's not, this branch introduces failures in aggregate failures scenarios but master is failing due to our cross repo checks which in turn are failing due to diff-lcs changes elsewhere.

[JonRowe]

Apologies, I checked again it is failing with the same error. However this cannot be merged until it is green.

[JonRowe]

Master is green if you want to rebase this

[MaxLap]

All green :)

[pirj]

Thanks!
Don't forget to add a changelog entry. No change is too small.