Fix problems with unVectorizeUnboxedX

Hi! I noticed a problem in `unVectorizeUnboxedX_` while using the library. The problem stemmed from calling unsafeCoerce on `v` as opposed to `pv` in `unVectorizeUnboxedX_`, and not fully constructing the UnsafePrimitiveArray for `pv` (failing to pass it `arr`). This was causing problems such as:
```
unVectorizeUnboxedX4 $ vectorizeUnboxedX4 $ VU.fromList [1,2,3,4]
fromList [1.0]
```
This pull request fixes this problem, and it now returns `fromList [1.0,2.0,3.0,4.0]` as it should.