Added HC-SR04 ultrasonic distance sensor. (#143)

* Added HC-SR04 ultrasonic distance sensor.

examples/hcsr04/main.go

@@ -0,0 +1,20 @@
+package hcsr04
+
+import (
+	"machine"
+	"time"
+
+	"tinygo.org/x/drivers/hcsr04"
+)
+
+func main() {
+	sensor := hcsr04.New(machine.D10, machine.D9)
+	sensor.Configure()
+
+	println("Ultrasonic starts")
+	for {
+		println("Distance:", sensor.ReadDistance(), "mm")
+
+		time.Sleep(100 * time.Millisecond)
+	}
+}