Using mutable and immutable vectors together is painful

This is actually 2 closely-related issues:
* Importing both `Data.Vector.Generic` and `Data.Vector.Generic.Mutable` in the same module means one must qualify all the slice and length ops, for example `take` and `drop`, tho these are effectively the same operations on mutable and immutable vectors.
* It is effectively impossible to write one term which works with both mutable and immutable vectors, for examples, all which i needed lately:
* `tails :: Vector v a => v a -> [v a]`
* `tailMay :: Vector v a => v a -> Maybe (v a)`
* `wrapSlice :: Vector v a => Int -> Int -> v a -> v a -- slice wrapping back to start if it goes too far`
* `windows :: Vector v a => Int -> v a -> [v a] -- list of all subvectors of given length`

It should be possible to use the same function to slice both mutable and immutable vectors. I propose this:
```
class Slice v where
length :: v a -> Int
unsafeSlice :: Int -> Int -> v a -> v a
```
and instances for all `Vector` and `MVector` types; i believe the other slicing functions can be defined in terms of these.

I can implement this, so let me know whether you approve.