Slices and views with asContiguous copies the whole parent tensor data

```Nim
import arraymancer

let a = ones[int](1000,2)
let b = a[0,_]
echo b.data.len

let c = a.unsafeSlice(0,_).asContiguous()
echo c.data.len
```

Outputs:
```
2000
2000
```

This is because of the deep seq copy, this is causing slowdown in axis iterators (used by reduce) and anywhere copying slices.