gpiostream: Declare full duplex Conn

[maruel]

There's currently conn.Conn.Tx() which works on bytes. We need the same but that works on bits and that is clocked on a predefined mechanism, like SPI modes 0~3, UART and PCM modes. This communication mode is only meant for bits based protocols, not byte based ones.

A superset of this functionality is to enable parallel outputs, that is, group multiple pins together and clock their output simultaneously.

[maruel]

The more I think about it, the more it could look like a simplified version of https://periph.io/x/periph/conn/spi#Packet

[maruel]

Ported to periph/conn#32.