Support for completely empty tensors
Currently:

```Nim
import ../src/arraymancer

var a = newTensorUninit[float32](0)

echo a.repr
```
```
[shape = [data = [0, 0, 0, 0, 0, 0, 0],
len = 1],
strides = [data = [1, 0, 0, 0, 0, 0, 0],
len = 1],
offset = 0,
storage = [Fdata = 0x103b5d048[]]]
```

Displaying will crash
```
test_uninit.nim(14)      test_uninit
display.nim(23)          $
p_accessors.nim(138)     disp2d
data_structure.nim(211)  dataArray
system.nim(2833)         sysFatal
Error: unhandled exception: index out of bounds [IndexError]
```