fix: enforce consistent evaluation order in emit

[ivov]

Lis evaluates expressions left to right, but it was possible for the emitted Go to read an indexed receiver, computed callee, slice bound, or assignment target after a sibling expession had already changed a value it depended on, leading to the wrong slot, fn or ptr being used at runtime. Hence this PR pins those operands into a temp before the sibling runs, so eval order matches the source order.

For example:

fn bump(i: Ref<int>) -> int {
  i.* = 1
  99
}

fn main() {
  let xss = [[10], [20]]
  let mut i = 0
  let ys = xss[i].append(bump(&i))  // i is 0 when xss[i] is chosen
  print(ys[0])                      // 10  (previously printed 20)
}

Source order says xss[i] is chosen while i is still 0, so the append targets xss[0] and ys[0] is 10. Previously the emitted Go ran bump(&i) first, bumping i to 1, so it appended to xss[1] and printed 20. The fix captures xss[i] into a temp before bump runs, so the receiver is locked in at the source-order moment.

[github-actions]

Released in lisette-v0.2.15