Lift the requirement of pre-shared keys of seed nodes

[kim]

Essentially, the membership protocol now knows that a peer is the tuple (node id, network address), which the handshake protocol is obliged to return us (or an error if it doesn't support establishing a channel without a PSK, and none is provided).

[kim]

There is one issue with this approach: it is no longer possible for the interpreter to extend the Peer type without also changing the membership code (or tying a conversion function into the state-keeping mechanism). It seems preferable to impose a weaker constraint on the HyParView monad:

class HasNodeId a where
    type NodeId a
    nodeId :: (Eq (NodeId n), Hashable (NodeId n)) => a -> NodeId a

class HasAddr a where
    type Addr a
    addr :: (Eq (Addr a), Hashable (Addr a)) => a -> Addr a

joinAny :: [(Maybe (NodeId n), Addr n)] -> HyParView n ()
joinAny = ...

[adinapoli-mndc]

@kim That seems reasonable -- typeclasses and type families to the rescue 😁

[kim]

Aww, but it's more complicated than that: I still need to specialise to some Peer nodeid addr, as otherwise we may unnecessarily hash a big structure for the HashSets/HashTables. So it'd be more the traditional "classy lenses".

[kim]

That last idea turned out to be a very elaborate way of implementing id, so I'm going with the "field lenses" approach. @adinapoli-mndc mind having another look?