Trim the search key from head as it's being pushed deeper into the trie. Previously the search key was never modified but each node kept information how to slice and compare it in keyOffset. Now the keyOffset is not needed as this information is included in the slice of the search key. This way the keyOffset can be removed and key manipulation
simplified.
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.
* trie: fix error in stacktrie not committing small roots
* trie: improved tests
* trie: fix error in stacktrie with small nodes
* trie: add (skipped) testcase for stacktrie
* trie: fix docs in stacktrie
