gpio: add Group interface, to act on multiple GPIO as a single transaction

[maruel]

Ported from google/periph#372.

Let's aim to have at least an idea how a gpio.Group would look like. A Group would be a logical group, not a physical one. To clarify, we're not talking about GPIOs grouped together on a header, I'm talking about GPIOs that are on the same controller thus can be acted on upon as a transaction. See the previous comment for precedents.

My thinking at the moment is to:

• Add gpio.Group interface, listing its owned pins.
• Add to interface gpio.Pin a new method .Group().

The interface Group would contain the transactional operations.