On ESP8266, there're 2 different interfaces. Pretending it's not the case
desn't make sense. So, network.WLAN() now takes interface id, and returns
interface object. Individual operations are then methods of interface
object. Some operations require i/f of specific type (e.g. .connect()
makes sense only for STA), other are defined for any (e.g. .ifconfig(),
.mac()).