Add `anyWord8` function to `Data.Attoparsec.Zepto`

I recently found use for a version of the `anyWord8` function defined over `Zepto`, which I implemented as:

```hs
import qualified Data.ByteString.Unsafe as BSU

anyWord8 :: Monad m => ZeptoT m Word8
anyWord8 = do
b <- Zepto.take 1
pure (BSU.unsafeHead b)
```

Since this requires using `Data.ByteString.Unsafe`, it would be nice to define this in `attoparsec` itself rather than in my downstream library (which would otherwise not need to make use of unsafe `ByteString` functions).