The approach using `matcherBelowDep : NameSet` was not correct because
we "reuse" matcher-declarations. For example, in the definition
```
def f : Nat → Bool
| 0 => true
| n + 1 => (match n with
| 0 => true
| _ + 1 => true) && f n
```
we have to `match`-expressions but they can be compiled the same
matcher `f.match_1`. Thus, the set `matcherBelowDep` would contain
`f.match_1` since the first occurence refined the `Nat.below` argument
at `mkBRecOn`. Thus, `addSmartUnfoldingDef` was incorrectly assuming the second
`match` was refined too.
We fixed this issue by simulating `mkBRecOn` behavior.
fixes#445
