Add proc -nomux to Yosys and migrate to it

[whitequark]

The position of Yosys developers (I say with my nMigen hat on) is that using proc_* passes in isolation is likely to break. We currently use them to prepare the netlist for write_verilog, which can't handle the netlist we emit as-is and requires it to be in a rather specific (and somewhat inconvenient) format.

What we do could almost certainly be replaced with proc -nomux (i.e. what the proc wrapper does minus proc_mux), which would be less fragile.

[whitequark]

proc -nomux is now available upstream; we need at least a node version bump to actually use it though.