[enhancement] Better codegen for XLS FIFOs (FFI)

[grebe]

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

Ideally, XLS codegen would have more customization around how it instantiates FIFOs, perhaps via a format string or whatever more sophisticated method we want to use more in the future.

Current best alternative workaround (limit 100 words)

Make a wrapper that uses the hardcoded name xls_fifo_wrapper.

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

The best case is that codegen can for each FIFO codegen the expected instantiation of a FIFO, e.g. if some are flop-based FIFOs and others are SRAM/BRAM-based, XLS codegen should be able to directly instantiate the desired FIFO.