pretty-printer does not parenthesize ret expr in callee position

```
(ret g)(41)
```

becomes

```
ret g(41)
```

Here's an invalid program that gets transformed into a program that prints 42.

```
fn f(g: fn(int) -> int) -> int { (ret g)(41) }
fn succ(x: int) -> int { x + 1 }
fn main() { log_err f(succ); }
```
