Fix memory unsafety in array_walk()

Fixes bugs #61967, #62607, #69068, #70713.

The primary changes are:
a) Use the ht_iterator mechanism to ensure safety not only if the

Fix memory unsafety in array_walk()

Fixes bugs #61967, #62607, #69068, #70713.

The primary changes are:
a) Use the ht_iterator mechanism to ensure safety not only if the
iterated array itself changes, but also if it is replaced (and
potentially destroyed) entirely. We use the same semantics for
behavior under modification as foreach-by-reference. In
particular, we advance to the next element before processing it.
If the iterated entity is exchanged we iterate the new one from
the start. If it is not an array/object we warn and abort.
b) Always create a reference to the current value. Previously the
code kept the value as a non-reference and updated it to the
reference value produced by the user callback. However this is
unsafe, as the array may have been reallocated in the meantime,
so the previous value pointer is no longer value.
c) Around a recursive walk, incref the reference containing the
array. This ensures that the location where the currently
iterated value is stored cannot be freed.

One problem I was not able to solve is that we cannot decrement
the apply count if the array is exchanged during a recursive walk.

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