Handle edge case where the same `Subsignal` uses multiple `Connectors`

[cr1901]

Occasionally, it is possible a user will want to add their custom Resource to their boards Connectors that matches the following conditions:

• A Subsignal of their shiny new Resource has multiple Pins that should be associated with it.
• The Pins of this Subsignal will be spread out across multiple Connectors.

omigen already handles this case by hardcoding the connector to use into the Pins string, as per this example on the b Subsignal.

What should the equivalent nmigen behavior be?

My Proposal

vga = [
        ("vga_out", 0,
            Subsignal("hsync", PinsN("3", dir="o", conn=("led", 0))),
            Subsignal("vsync", PinsN("4", dir="o", conn=("led", 0))),

            Subsignal("r", Pins("1 2 3", dir="o", conn=("dio", 0))),
            Subsignal("g", Pins("4 5 6", dir="o", conn=("dio", 0))),
            Subsignal("b", Pins("7", dir="o", conn=("dio", 0)),
                           Pins("1", dir="o", conn=("clkio", 0))),
            Attrs(IOSTANDARD="LVCMOS33", SLEW="FAST")
        )
    ]

[whitequark]

That's already handled by doing something like:

vga = [
        ("vga_out", 0,
            Subsignal("hsync", PinsN("led_0:3", dir="o")),
            Subsignal("vsync", PinsN("led_0:4", dir="o")),

            Subsignal("r", Pins("dio_0:1 dio_0:2 dio_0:3", dir="o")),
            Subsignal("g", Pins("dio_0:4 dio_0:5 dio_0:6", dir="o")),
            Subsignal("b", Pins("dio_0:7 clkio_0:1", dir="o")),
            Attrs(IOSTANDARD="LVCMOS33", SLEW="FAST")
        )
    ]