Cleaner nextpnr BBA generation

Currently the nextpnr BBA generation code includes a lot of duplication, particularly where lists are concerned. e.g. patterns like https://github.com/SymbiFlow/python-fpga-interchange/blob/6a80233ca97427c9b1a1cdd16349c3724914d008/fpga_interchange/chip_info.py#L658-L672 crop up a lot

I think a custom decorator might be the nicest solution here, although I don't have enough Python experience personally to comment on how much work it would be to implement this.

Longer term, the structures in nextpnr (https://github.com/YosysHQ/nextpnr/blob/master/fpga_interchange/chipdb.h) and Python should come from the same source to reduce the risk of mismatches, although I don't know which should be definitive.