Add conn.Conn.Duplex() (#118)

Now each connection can report if the underlying communication channel is
half-duplex or full-duplex.

This information is leveraged by some clients, like memory mapped registers
helper struct, to change their behavior or deny use on certain kind of
communication channel.

experimental/devices/bitbang/spi.go

@@ -17,6 +17,7 @@ import (
 	"sync"
 	"time"
 
+	"periph.io/x/periph/conn"
 	"periph.io/x/periph/conn/gpio"
 	"periph.io/x/periph/conn/spi"
 	"periph.io/x/periph/host/cpu"
@@ -44,6 +45,12 @@ func (s *SPI) Close() error {
 	return nil
 }
 
+// Duplex implements spi.Conn.
+func (s *SPI) Duplex() conn.Duplex {
+	// Maybe implement bitbanging SPI only in half mode?
+	return conn.Full
+}
+
 // Speed implements spi.Conn.
 func (s *SPI) Speed(hz int64) error {
 	s.mu.Lock()