`IdleCircuit` should be split up

[martijnbastiaan]

IdleCircuit is currently defined as:

{- | Idle state of a Circuit. Aims to provide no data for both the forward and
backward direction. Transactions are not acknowledged.
-}
class (Protocol p) => IdleCircuit p where
  idleFwd :: Proxy p -> Fwd (p :: Type)
  idleBwd :: Proxy p -> Bwd (p :: Type)

I.e., it forces instances to define both their Fwd and Bwd to be defined simultaneously. This doesn't make sense for protocols such as CSignal. I think it should be split up in:

class (Protocol p) => IdleFwd p where
  idleFwd :: Proxy p -> Fwd (p :: Type)

class (Protocol p) => IdleBwd p where
  idleBwd :: Proxy p -> Bwd (p :: Type)

idleSource :: forall p. (IdleFwd p) => Circuit () p
idleSource = Circuit $ const ((), idleFwd $ Proxy @p)

idleSink :: forall p. (IdleBwd p) => Circuit p ()
idleSink = Circuit $ const (idleBwd $ Proxy @p, ())

[martijnbastiaan]

By the by, I also think the naming of the functions isn't quite right. Sink and source assume too much about the protocol. It really is just "left" and "right".