conn: Simplify Conn, rename i2c.Conn to i2c.Bus. (#40)

- Remove fmt.Stringer and io.Writer from conn.Conn. This makes it compatible
  with exp/io.
- Rename i2c.Conn to i2c.Bus since this was confusion. This clarifies the
  distinction between a point-to-point connection and a bus.
- Add these to bus specific interfaces, i2c.Bus, spi.Conn, uart.Conn.

Fixes #15.

devices/bme280/bme280.go

@@ -137,7 +137,7 @@ type Opts struct {
 //
 // It is recommended to call Stop() when done with the device so it stops
 // sampling.
-func NewI2C(i i2c.Conn, opts *Opts) (*Dev, error) {
+func NewI2C(b i2c.Bus, opts *Opts) (*Dev, error) {
 	addr := uint16(0x76)
 	if opts != nil {
 		switch opts.Address {
@@ -149,7 +149,7 @@ func NewI2C(i i2c.Conn, opts *Opts) (*Dev, error) {
 			return nil, errors.New("given address not supported by device")
 		}
 	}
-	d := &Dev{d: &i2c.Dev{Conn: i, Addr: addr}, isSPI: false}
+	d := &Dev{d: &i2c.Dev{Bus: b, Addr: addr}, isSPI: false}
 	if err := d.makeDev(opts); err != nil {
 		return nil, err
 	}
@@ -294,8 +294,7 @@ func (d *Dev) writeCommands(b []byte) error {
 			b[i] &^= 0x80
 		}
 	}
-	_, err := d.d.Write(b)
+	return d.d.Tx(b, nil)
-	return err
 }
 
 // Register table:

devices/ssd1306/ssd1306.go

@@ -88,8 +88,8 @@ func NewSPI(s spi.Conn, w, h int, rotated bool) (*Dev, error) {
 //
 // As per datasheet, maximum clock speed is 1/2.5µs = 400KHz. It's worth
 // bumping up from default bus speed of 100KHz if possible.
-func NewI2C(i i2c.Conn, w, h int, rotated bool) (*Dev, error) {
+func NewI2C(i i2c.Bus, w, h int, rotated bool) (*Dev, error) {
-	return newDev(&i2c.Dev{Conn: i, Addr: 0x3C}, w, h, rotated)
+	return newDev(&i2c.Dev{Bus: i, Addr: 0x3C}, w, h, rotated)
 }
 
 // newDev is the common initialization code that is independent of the bus

experimental/devices/bitbang/i2c.go

@@ -35,12 +35,12 @@ func (i *I2C) String() string {
 	return fmt.Sprintf("bitbang/i2c(%s, %s)", i.scl, i.sda)
 }
 
-// Close implements i2c.ConnCloser.
+// Close implements i2c.BusCloser.
 func (i *I2C) Close() error {
 	return nil
 }
 
-// Tx implements i2c.Conn.
+// Tx implements i2c.Bus.
 func (i *I2C) Tx(addr uint16, w, r []byte) error {
 	i.mu.Lock()
 	defer i.mu.Unlock()
@@ -88,7 +88,7 @@ func (i *I2C) Tx(addr uint16, w, r []byte) error {
 	return nil
 }
 
-// Speed implements i2c.Conn.
+// Speed implements i2c.Bus.
 func (i *I2C) Speed(hz int64) error {
 	i.mu.Lock()
 	defer i.mu.Unlock()
@@ -254,4 +254,4 @@ func (i *I2C) sleepHalfCycle() {
 	host.Nanospin(i.halfCycle)
 }
 
-var _ i2c.Conn = &I2C{}
+var _ i2c.Bus = &I2C{}