Provide interesting example programs

[quchen]

• Associativity of list concatenation: left is quadratic, right is linear

• Naive repeat/cycle vs version with good sharing

• Naive Fibonacci: fib n = fib (n-1) + fib (n-2)

• Improved, linear Fibonacci

  fib = fib' 0 1
    where
      fib' x _ | n <= 0 = x
      fib' x !y n = fib' y (x+y) (n-1)

• foldl vs foldl-via-foldr