Warnings cleanup

Hi, 

there were some warnings during compilation, so I fixed them. I removed two redundant imports and I also added ```-fno-warn-unused-do-bind``` to prevent warnings from constructs such as this:

```haskell
percentval = do
             string "%%"
             return "%"
```

An alternative is to write:
```haskell
percentval = do
             _ <- string "%%"
             return "%"
```

Which is more verbose but explicitly shows that the binding is discarded. If you think that this is better than adding ```-fno-warn-unused-do-bind``` I can modify my patch.