Improve fuzzing in the {K,H,}AMT

We have fuzzers for the KAMT, HAMT, and AMT but these fuzzers only test set/get/remove/flush. Importantly, they don't test:

1. Flush and reload from CID (just periodic flushes).
2. `set_root`.
3. Iteration.
4. Clearing.

See https://github.com/filecoin-project/ref-fvm/blob/master/ipld/kamt/fuzz/fuzz_targets/common.rs (and the same for the hamt and amt).