Span slicing inside dynamic type procs fails to compile
Minor test case:

```Nim
import arraymancer

proc boo[T](): T =
  var a = zeros([2,2], int)
  echo a[1,_]

discard boo[int]()
```

When compiling the following error is shown:
```
test.nim(4, 12) Error: undeclared identifier: '_'
```

Removing dynamic type `[T]` from the proc the code works fine.