trie: make stacktrie not mutate input values (#22673)

The stacktrie is a bit un-untuitive, API-wise: since it mutates input values.
Such behaviour is dangerous, and easy to get wrong if the calling code 'forgets' this quirk. The behaviour is fixed by this PR, so that the input values are not modified by the stacktrie. 

Note: just as with the Trie, the stacktrie still references the live input objects, so it's still _not_ safe to mutate the values form the callsite.