Bars (|) inside brackets mess up highlighting

Currently the logic here 

https://github.com/theindigamer/ats2-mode/blob/d08ac0287a5ac7161a856520dcddb6a012f07d23/ats2-mode.el#L180-L190

means that when you have a situation like
```
(
  case foo of
  | bar ...
```
the region `case foo of |` will get highlighted using the wrong face which doesn't make sense. The highlighting should only apply to proofs.