expression drill-down gone?

while the diffs are super helpful, it looks like it loses the very handy drill-down features:

```
def test_a():
assert len([1, 2, 3]) == len([1, 2])
```

without plugin
```
test_foo.py:2: in test_a
assert len([1, 2, 3]) == len([1, 2])
E assert 3 == 2
E + where 3 = len([1, 2, 3])
E + and 2 = len([1, 2])
```

with plugin
```
test_foo.py:2: in test_a
assert len([1, 2, 3]) == len([1, 2])
E assert equals failed
E 3 2
```