a simplified version of a Function Unit pipeline is needed which has
the same functionality as soc.experiment.alu_hier.ALU but has the
soc.fu front-end API (including regspec compliance).

take this code (particularly main_stage.py):
https://git.libre-soc.org/?p=soc.git;a=tree;f=src/soc/fu/alu;hb=HEAD

and cut it *right* back so that it has the same functionality as this:
https://git.libre-soc.org/?p=soc.git;a=blob;f=src/soc/experiment/alu_hier.py;h=bebe6468cc78975e0e98794fe8d5f72321eb0d50;hb=HEAD#l195

but without the mess of the actual ALU side being mixed in with
the pipeline handling.