fix up for more

In master, we currently have:

```Clojure
=> (for [x (range 2) y (range 2)] (print x))
```

Which generates:

```Python
for x in range(2L):
    for y in range(2L):
        print(x)
```

This sucks. I hear @nedbat screaming at me in my head. I couldn't sleep.

So, I fixed our macro. Now, given the same expression, Hy will produce:

```Python
import itertools
for (x, y) in itertools.product(range(2), range(2)):
    print(x)
```

Nom.