Fix stateful expressions in UDFs

[mattnibs]

This commit fixes an issue with using aggregation expressions user-defined functions where there wasn't a separate state per textual invocation.

Closes #5092

[nwt]

Do these two programs produce different output by design?

$ echo 0 1 0 1 | zq '
  func f(x): (
    x == 0 ? count() : count()
  )

  yield f(this)
' -
1(uint64)
1(uint64)
2(uint64)
2(uint64)

$ echo 0 1 0 1 | zq '
  func my_count(): (
    count()
  )

  func f(x): (
    x == 0 ? my_count() : my_count()
  )

  yield f(this)
' -
1(uint64)
2(uint64)
3(uint64)
4(uint64)

[mattnibs]

Do these two programs produce different output by design?

This seems like a bug to me. Looking into it.

EDIT

Good catch, I think this highlights a limitation of my approach. Going to have to think about this one.