[NFC] Clarify and standardize order in flexibleCopy

[kripken]

flexibleCopy always visited parents before children, but it visited
vector children in reverse order:

(call        ;; 1
  (call $a)  ;; 3
  (call $b)  ;; 2
)

The order of children happened to not matter in any user of this code,
and that's just what you get when you iterate over children in a vector
and push them to a stack before visiting them, so this odd ordering
was not noticed.

For a new user I will introduce soon, however, it would be nice to have
the normal pre-order:

(call        ;; 1
  (call $a)  ;; 2
  (call $b)  ;; 3
)

(2 & 3 swapped).

This cannot be tested in the current code as it is NFC, but the later PR
will depend on it and test it heavily.