Chapter 13, Testing Polymorphic Code

This 
```PureScript
ints :: Array Int -> Array Int
ints = id
```
should be
```PureScript
ints :: Array Int -> Array Int
ints = identity
```

Also for:
```PureScript
intToBool ∷ (Int → Boolean) → Int → Boolean
intToBool = identity

treeOfInt ∷ Tree Int → Tree Int
treeOfInt = identity
```

