instant seems to have no effect when wrapped around definitions

[byorgey]

Describe the bug
instant, when wrapped around a term that includes some expensive defs, seems to have no effect.

To Reproduce
In the below scenario, the judge does an expensive computation (naive fib 10), then places a rock. The base robot just executes move; grab. Since the entire judge program is wrapped in instant, this ought to work: the entire judge program including the expensive computation and the place command should all execute in a single tick, placing the rock in time for the base to grab it. However, the base crashes with a "nothing here to grab" error, and the rock shows up later.

version: 1
name: Instant wrapped defs
description: |
  `instant` should work when wrapped around definitions
creative: false
objectives:
  - goal:
      - Grab the rock
    condition: |
      as base { has "rock" }
robots:
  - name: base
    dir: east
    devices:
      - logger
      - treads
      - grabber
  - name: judge
    dir: east
    system: true
    program: |
      instant (
      def fib : Int -> Int = \n.
        if (n <= 1) {n} {fib (n-1) + fib (n-2)}
      end

      def x = fib 10 end

      create "rock"; place "rock"
      )
solution: |
  move; grab
known: [rock]
world:
  dsl: |
    {grass}
  palette:
    'B': [grass, null, base]
    'j': [grass, null, judge]
  upperleft: [0, 0]
  map: |
    Bj

Additional context
Ran into this while preparing #2260. See the discussion here: #2260 (comment)

[byorgey]

Ohhhh, I understand why this happens: it's because Swarm is a strict language, so the argument to instant is evaluated first, before passing it to instant! In this case, evaluating the argument includes evaluating the values of the definitions.

So this is not exactly a bug, but it is confusing, and we might want to consider special-casing it somehow...