Persistent State for HashMap::compute

[ibraheemdev]

Currently HashMap::compute requires an FnMut and has no way of persisting state. For example, inserting a V requires cloning if the operation fails and is retried. Internally we memoize all inserts to avoid this, but that's a hack that shouldn't be expose to users. Instead we might want to provide an on_failure(Operation<V>) hook to allow state to be reset. I don't want the API to become convoluted, so maybe just guaranteeing that inserts are memoized is simpler?