DRC: edge layer - merged method gives an error

The following script

```
l1 = input(1, 0)
l1.edges.merged
```

(1/0 is a single box) gives this error:

```
Can't match arguments. Variants are:
  const Edges merged() const [0 argument(s) expected, but 1 given]
 in Edges::merged
```

Reason is that `Edges#merged` does not have an overlap count.