udf fn passed as arg to udf op

I thought this was a cool example found in the superdb sources:

https://github.com/brimdata/super/blob/6f6ff281d8a49682513620d769d94df95217f731/compiler/semantic/ztests/func-arg-to-op.yaml

```
spq: |
  fn g(x) : (x+1)
  op foo f: (values f(123))
  foo g

output: |
  124
```