Complete top-down evaluation implementation

The following does not pass with top-down evaluation:

```rego
package test
import future.keywords

p[q].r contains s {  # Rule with prefix and general ref
   x := ["a", "b", "c"]
   q := x[_]
   s := x[_]
   q != s
}
p.b.r contains "foo" # Rule with prefix overlapping with above rule
```

Query: `data.test.p.b`