[enhancement] Codegen should have a default FIFO implementation

[grebe]

What's hard to do? (limit 100 words)

Currently you need to provide a FIFO implementation or set the --materialize_internal_fifos flag if you use multi-proc codegen or loopback channels. Without doing one of these things, you'll have FIFO instantiations with no module definitions.

Related: #1667.

Current best alternative workaround (limit 100 words)

Provide a FIFO implementation or set --materialize_internal_fifos.

Your view of the "best case XLS enhancement" (limit 100 words)

We should move our default to --materialize_internal_fifos or provide a good default FIFO implementation.

[ericastor]

This got a bit worse after commit 4b262b0, since now this affects even depth=0 FIFOs; in fact, a few examples in this repository broke without us noticing, including our RAM-based delay example!