DV.Generic.Mutable.fill does not check for out of bounds access

So it should be either renamed as unsafeFill and/or bound checks added. It's possible this is not only one function with unsafe access.

``` haskell
module Fill where

import qualified Data.Vector.Unboxed.Mutable as MU
import qualified Data.Vector.Generic.Mutable as MG
import qualified Data.Vector.Fusion.Stream.Monadic as SS

main :: IO ()
main = do
mv <- MU.replicate 10 (1 :: Int)
MG.fill mv (SS.replicate 1000000 42)
return ()
```

```
[1 of 1] Compiling Fill ( /home/alexey/qqq/fill-sigsegv.hs, interpreted )
Ok, modules loaded: Fill.
*Fill> :main
Segmentation fault
```