v0.0-986-g24a8fe0: formatter: Fix constraints blocks formatting (PR #706)

Expands constraints blocks with more than one partition, e.g. if-statements
```
constraint xx {
  if (a) b;
}
```

But keeps constraints blocks with simple statements unexpanded, e.g.

```
constraint only_vec_instr_c {soft only_vec_instr == 0;}
```

Fixes #445