[enhancement] Codegen should have a default FIFO implementation

### 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.