I don't really like this heuristic.  For example, this expression would definitely be better with Parakeet:

```
x = ones(1000)
for i in range(20):
  x = x + x
```

But it doesn't contain any function calls.

(On the other hand, I don't have any better ideas at the moment).  Can we do 2 things?  First, let's split this out into a separate function:

```
def _should_run_parakeet(expr):
...
```

Second, let's make the heuristic a bit more broad: if it has a function call, OR the number of deps is greater than, say, 2, we go into Parakeet?