fix compilation error for 05.answer.scala in chapter 12
Update 05.answer.scala
Update 04.hint.txt
Update 05.hint.txt
Update 07.hint.txt
Update 13.hint.txt
Merge pull request #341 from psliwa/patch-1

fix compilation error for 05.answer.scala in chapter 12
Merge branch 'master' of github.com:fpinscala/fpinscala