Add an explanation header to the difference summary

[bjacotg]

This will produce the following result, which makes more obvious what colors and +/- means.

This triggered the following refactoring:

• Don't rely on left and right to describe the argument of edit_list.
• Accept actual as first argument and expected as second (like in verify_that!)
• Use Red vs Green instead of Red vs Blue for better contrast.
• Use - for actual and + for expected. In a TTD process, expected is the ground truth that the code should generate, hence it reads as the recommendation: The code should remove the -lines and generate the+` lines.