Add a fuzzer

[irevoire]

Pull Request

Add a very basic fuzzer on arroy:

• The fuzzer is located in examples/fuzz.rs
• I added the fuzzer in the CI, and it runs for 5 minutes
• While fuzzing arroy, I found and fixed two bugs on the incremental indexing PR that was just merged:
  • Calling build when there are no vectors in the database was triggering a division by zero in the remove_extraneous_trees method
  • In some cases, the newly crafted left and right children of a split node were not in the two last positions in the tmp_nodes. Now we check the whole list of tmp_nodes when we need to delete something, but still from the end since it's related to the latest nodes creations.