Nonzero performance bump

Tested using current AG table (21k observations, 4.5k samples). Old `nonzero`:

```python
> %timeit list(t.nonzero())
1 loops, best of 3: 5.98 s per loop
```

New `nonzero`:

```python
> %timeit list(t.nonzero())
1 loops, best of 3: 592 ms per loop
```