chapter 6.8 in version 12.0

```
instance streamString :: Stream String Char where
  uncons = String.uncons
```

Failed to compile with error "Could not match type CodePoint with type Char"
I have to replace that code into
```
instance streamString :: Stream String Char where
    uncons "" =  Nothing
    uncons s = Just {head: charAt 0 s, tail: String.drop 1 s}
```
to make it work