gpioreg: Remove ByNumber() (#153)

It is still possible to get a pin by its logical number using the string
representation, but the function is not exported anymore.

This simplifies the interface, one less way to do the same thing.

devices/tm1637/tm1637_test.go

@@ -20,7 +20,7 @@ func Example() {
 	if _, err := host.Init(); err != nil {
 		log.Fatalf("failed to initialize periph: %v", err)
 	}
-	dev, err := New(gpioreg.ByNumber(6), gpioreg.ByNumber(12))
+	dev, err := New(gpioreg.ByName("GPIO6"), gpioreg.ByName("GPIO12"))
 	if err != nil {
 		log.Fatalf("failed to initialize tm1637: %v", err)
 	}