[RFC] Merge FifoCommandRunnable into Decode

No idea if this change is desirable.
I personally find having function parameters that toggle different behavior annoying, because they add another roundtrip when trying to understand the code ("oh hey, this place seems to decode the next command.. but uh, what's that parameter used for?").

OTOH, I don't know if there's much we can do about this. Maybe make it a strongly typed enum instead? :/