Allow lazy expression slices in assignments

The same way that lazy expressions are evaluated in getitem contexts like:

```
res = iarr2[iarr >= 0.5].eval()
```

the next should be possible too:

```
iarr2[iarr >= 0.5] = 1
```

the result will be `iarr2` values being 1 when the `iarr >= 0.5` condition is fulfilled, and NaNs elsewhere.