Bus indices should (arguably) be signed not unsigned

[gatecat]

Based on some recent comments in YosysHQ/yosys#2567 it is apparent that negative bus indices are legal Verilog (and occasionally used for the fractional part of fixed point values) and therefore should be representable in the interchange format too. This would mean using a signed integer type for these.

[litghost]

Sure, this make sense.