Ex. 1 and 2.
Merge branch 'master' of https://github.com/aenache/fpinscala

# By Rúnar (5) and psliwa (1)
# Via Rúnar
* 'master' of https://github.com/aenache/fpinscala:
  Update 13.hint.txt
  Update 07.hint.txt
  Update 05.hint.txt
  Update 04.hint.txt
  Update 05.answer.scala
  fix compilation error for 05.answer.scala in chapter 12