Feature request: allow si.bus(0)

[Bart Brouns]

I have encountered a couple of times a situation where it would be handy to have a si.bus(0).
I've implemented it like so:

bus(0) = 0:!;
bus(2) = _,_; // avoids a lot of "bus(2)" labels in block diagrams
bus(n) = par(i, n, _);

Alternatively, it might be a better idea to allow par(i,0,expression), but I have no idea if there are any downsides to that.

I'd be more than happy to do a PR, once you decide what you want.

[Bart Brouns]

I implemented the first suggestion as part of this PR.