Use a progress bar indicator to inspect BC breaks

[Ocramius]

As discussed with @asgrim in https://github.com/Roave/BackwardCompatibilityCheck/issues/77:

That's acceptable I think... Or maybe simply collect errors and display at the end? Thinking a summary output like phpunit/phpcs like:

.......B......E......
.....B............EEE

Where B is a break and E is an error.. then display them at the end?

This means that array is not a good solution for accumulating BC breaks, and instead yield and yield from must be used aggressively to iterate over the results as they get produced, and inform the user about each step in the process by continuing to produce output.

[schmunk42]

A progress bar would be really helpful, I am trying to check https://github.com/yiisoft/yii2 and in my last run it took about 5min, just to tell me that I do not have enough memory :)

[Ocramius]

@schmunk42 wanna give this a shot, maybe? Also, I think most of the speed problem comes from using the wrong type of SourceLocator ( https://github.com/Roave/BackwardCompatibilityCheck/issues/110 )

[schmunk42]

To be honest, I don't think I can find time in the near future; so if someone else is planning to do this: "don't hesitate"

[Ocramius]

Deferring this: current implementation now uses generators as per #108, which makes the output less painful to wait for.