Well, word of warning: the MCP23017 is definitely not indestructible. Second, NEVER hook up something that SUPPLIES 5v to a sainsmart 16 relay module unless the sainsmart is being supplied with 12v. The 5v will flow at high currents, seemingly powering the onboard regulator and making things "kind of" work for a bit, then the regulator gets hot, etc. It was bad.
Read the datasheet, not just the pinout :D
Here's the final protoboard, which ended up toast:
You can see where I severed the 5v connection in an attempt to salvage things. Fortunately I got 5 of the MCP23017, so we're not doomed yet!.
Subsequently, however, I put things on a breadboard:
And finally, I wrote up some go code to make clicky-blinky things happen!