Remove Semigroup (Stream a); it's not associative.

```haskell
*S11> take 10 $ repeat 1 <> (repeat 2 <> repeat 3)
[1,2,1,3,1,2,1,3,1,2]
*S11> take 10 $ (repeat 1 <> repeat 2) <> repeat 3
[1,3,2,3,1,3,2,3,1,3]
```

Given

```haskell
module S11 where

import Prelude hiding (repeat, take)
import Data.Stream.Infinite
import Data.Semigroup
```