`Vector` constructor is not exposed.

I find it slightly confusing.

`Data.Vector.Primitive.Vector` constructor is exposed, but for `Data.Vector` it isn't. The inconsistency doesn't make sense to me, and I'd like to have internals (i.e. access to size and offset) exposed *somewhere*.

@lehins pointed somewher else (Reddit?) that `quickcheck-instances` doesn't generate sliced `Vector`s. But from current API of `Vector` it doesn't seem to matter, as far as I can see, there isn't a way to exploit slicedness of ordinary `Vector`s, (you can exploit slicedness of some other vectors, but not all).