API revamp: Pluggable hash map and more iterators

[Knut Magnus Aasrud]

Vault would be very ergonomic if it had custom iterators like Keys for e.g. the backlinks function - perhaps named NoteIds - and similarly for iter, into_iter, etc.

Rewrite the current HashMap-implementation of mdzk's adjacency matrix to use a general hash map defined outside Vault. Doing this would allow easily exchanging what hashing function we want used - since we don't really need a cryptographic one for our use-case.

Additionally, go over the naming convention. Perhaps backlinks should be replaced with incoming_links and then the corresponding links function in Note should be named outgoing_links?