Improve solutions in chapter 2

[pitmonticone]

• Make solutions more structured and readable
• Make finishing tactics more explicit

These changes are mostly based on the feedback I received from participants of the Lean Tutorial in Vienna.

[avigad]

I expect that we used apply here rather than exact because we just introduced apply and we wanted the reader to get used to it. We'll re-evaluate this PR when we make another proofreading pass through MIL.