periph: Remove Type; gpio: improve pin registering (#48)

periph:
- Type was an approximation to enforce priority ordering. Remove the need for
  approximate ordering by improving gpio instead to support out-of-order pin
  registration. This has the side effect of increasing parallelism. This saved
  ~5ms on "gpio-list" runtime (out of 30ms) when run on a Raspberry Pi 3.

gpio:
- Redo RegisterAlias() to support out of order registration.
- Replace Functional() with Aliases(), which is much more generic.
- Unexport PinAlias, it is not necessary anymore.
- Improve error messages by wrapping the error, it helps legibility in case of
  errors.
- Remove Unregister(), it will be added back when there's an actual call site.
- Remove the (%d) number on sysfs-gpio and bcm283x since it wasn't useful.
- sysfs-gpio: Fix edge when switching output.
- consistently fix accumulated edged when calling sysfs.Pin.In().

lirc:
- Stop mapping IR_IN and IR_OUT when the pin is not mapped.

periph-smoketest:
- Fix to use host.Init(), otherwise drivers weren't correctly loaded.

Fixes #45
(I think)

devices/lirc/lirc.go

@@ -14,7 +14,6 @@ import (
 	"strings"
 	"sync"
 
-	"github.com/google/periph"
 	"github.com/google/periph/conn/gpio"
 	"github.com/google/periph/conn/ir"
 )
@@ -215,33 +214,20 @@ func (d *driver) String() string {
 	return "lirc"
 }
 
-func (d *driver) Type() periph.Type {
-	// Return the lowest priority, which is Functional.
-	return periph.Functional
-}
-
 func (d *driver) Init() (bool, error) {
 	in, out := getPins()
 	if in == -1 && out == -1 {
 		return false, nil
 	}
 	if in != -1 {
-		if pin := gpio.ByNumber(in); pin != nil {
-			gpio.MapFunction("IR_IN", pin)
-		} else {
-			gpio.MapFunction("IR_IN", gpio.INVALID)
+		if err := gpio.RegisterAlias("IR_IN", in); err != nil {
+			return true, err
 		}
-	} else {
-		gpio.MapFunction("IR_IN", gpio.INVALID)
 	}
 	if out != -1 {
-		if pin := gpio.ByNumber(out); pin != nil {
-			gpio.MapFunction("IR_OUT", pin)
-		} else {
-			gpio.MapFunction("IR_OUT", gpio.INVALID)
+		if err := gpio.RegisterAlias("IR_OUT", out); err != nil {
+			return true, err
 		}
-	} else {
-		gpio.MapFunction("IR_OUT", gpio.INVALID)
 	}
 	return true, nil
 }