trie, tests/fuzzers: implement a stacktrie fuzzer + stacktrie fixes (#21799)

* trie: fix error in stacktrie not committing small roots

* fuzzers: make trie-fuzzer use correct returnvalues

* trie: improved tests

* tests/fuzzers: fuzzer for stacktrie vs regular trie

* test/fuzzers: make stacktrie fuzzer use 32-byte keys

* trie: fix error in stacktrie with small nodes

* trie: add (skipped) testcase for stacktrie

* tests/fuzzers: address review comments for stacktrie fuzzer

* trie: fix docs in stacktrie