partial eval: comprehensions not plugged after partial eval

[tsandall]

If a comprehension closes over local variables, they remain in the saved comprehension after being returned to the caller. As a result, when the caller receives a safety error when they try to compile the comprehension.

Example:

package test

p {
  x = 1
  [true | x]  # close over local var 'x'
}

Partially evaluating the query data.test.p results in:

[true | x] = _; neq(_, false); _ = true

Where x is unsafe.

The query evaluator should plug the comprehension before returning it to callers.