Add simpler PacketStream FIFO

[t-wallet]

Sometimes all you want to achieve with a FIFO is absorb minimal backpressure. Not having to buffer full packets allows for a shallower FIFO and less logic. The implementation can be largely copied from packetFifoC, removing all of the dropping semantics. Note that using DfConv.fifo is also a solution, but highly suboptimal because it does not have a separate FIFO for metadata.

Expected behaviour

• Dropping all timing information, the output data should be identical to the input data.
• Unlike packetFifoC, there may be gaps (Nothings) in output packets.
• The FIFO does not drop aborted packets.

The circuit is expected to have the following type:

fifoC ::
  forall dataWidth meta dom contentDepth metaDepth.
  SNat contentDepth ->
  SNat metaDepth ->
  Circuit (PacketStream dom meta dataWidth) (PacketStream dom meta dataWidth)