Detect and disallow invalid connections in JSON

If the input JSON has a connection between different-sized ports, DigitalJS will happily connect them, with disastrous results. For example, yosys2digitaljs currently generates such a bad JSON for:
```verilog
module test2(input [1:0] i, output o);
  assign o = i;
endmodule
module test(output o);
  test2 x(0'b1, o);
endmodule
```