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Now that "key == Qunbound" is used to determine if a hash table entry
is available, we can't stash pre-allocated vectors into the `key` slot
anymore, so use the `value` slot instead.

(make_log): Pre-fill the `value` slots i.s.o `key`.
(evict_lower_half): Stash key back into `value`, i.s.o `key`.
(record_backtrace): Get pre-allocated vector for `value` i.s.o `key`.