Support for MCP23016

[Flameeyes]

The MCP23016 is another member of the MCP230xx family, and a predecessor of the MCP23017 that is already supported.

The main differences between the '16 and the '17 (from the CircuiPython point of view), from what I can tell are:

• '16 has no pull up resistor configuration (afaict it's always in pull-up mode);
• '16 doesn't allow atomic operations on the whole 16 I/O lines, instead they are accessed as two 8-bit groups (the same is true for polarity configuration and interrupt config).

Otherwise it's pretty much business as usual from the software side. It's a bit more complicated on the hardware side, which I guess is why they don't recommend it for new hardware projects.

[Flameeyes]

Actually, I'm not sure about the '16 not supporting 16-bit atomic access! I checked the datasheets again side to side, and it looks like the GPIO0 and GPIO1 registers are side-to-side explicitly so that they can be used together…

[Flameeyes]

Confirmed that the '16 access both ports when writing a 16-bit value register. So the only difference between the '16 and the '17 that we need to care for is that the '16 doesn't have pull-up configuration (and a few interrupt controls).

Pull request #29 has all that is needed for the mcp23016 to be used.