[Haskell] Peano numbers behave well, Peano RPN calculator doesn't pass
the quickcheck
[Haskell] RPN calculator passes the quickcheck

- "pick" counted from Succ(Zero) rather than from Zero (instructions say
  "nth element", question is whether the "n" token should be taken into
account)
- "/" wasn't implemented
- "-" took its operands in reverse order