feat: add fuzz integration [take two]

[Davidson-Souza]

Partially replaces #92
Replaces #149

This pull request adds fuzzing capabilities to rustreexo, including targets to test our cryptographic code, focusing on stump — since this is the implementation we currently use in floresta.

The first target tests proof and stump [de]serialization, making sure it doesn't panic even on malformed inputs. The second creates intentionally invalid proofs and checks how our stump behaves.

These targets were initially written by Bruno in #149, I just cleaned them up and fixed problems found while running them.

[luisschwab]

Could add a fuzz CI job that runs $n$ iterations, just to verify the fuzzers work, and a justfile recipe so we can fuzz locally.