Add simpler PacketStream FIFO

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 (`Nothing`s) in output packets.
- The FIFO does not drop aborted packets.

The circuit is expected to have the following type:

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

Start by locating the existing `packetFifoC` implementation and the `DfConv.fifo` alternative referenced in the issue. Compare their handling of packet data, metadata, gaps, timing information, and aborted packets; done means a circuit with the specified `fifoC` type that preserves data while allowing output `Nothing`s and never drops aborted packets.

Written by the indexing model from the issue text.