feat: keyed streams

[MingweiSamuel]

Right now "keyed" streams are just regular streams of (K, V) pairs. This comes up in many situations, for example when handling message requests, the client addr is the key for ingress and egress. We don't really do anything with the K but we have to carry it around and it messes up types for things (like fold/reduce become fold_keyed/reduce_keyed). If we have proper keyed streams, or some sort of metadata wrapping stream items, could be powerful

@shadaj If you have more thoughts

[shadaj]

In particular, a separate type for keyed streams will make our ordering types richer. Currently, messages from a cluster are received as (ClusterID, Data) pairs. Even if the senders guarantee deterministic ordering, the interleaving results in the entire resulting stream being unordered. With a separate KeyedStream type, keys will always be unordered, but the ordering type declares if the elements within each group have deterministic order, which would capture more information.