Fix storages

[pnevyk]

46 introduced fuzz targets for all currently implemented storages. All of them failed. Fix the storages such that fuzzing does not reveal a problem after running for some time.

• [x] AdjList undirected
• [x] AdjList directed
• [x] AdjMatrix undirected
• [x] AdjMatrix directed
• [x] EdgeList undirected
• [x] EdgeList directed
• [x] Stable<AdjList> undirected
• [x] Stable<AdjList> directed

[pnevyk]

Fixing Stable storage is currently blocked by #55.

[pnevyk]

Completed in #61