servo: add driver using PWM

This PR adds support for controlling servos. For example, on the Arduino
Uno it should be able to control up to 6 servos jitter-free when using
all available PWM pins.

I haven't added support for setting a position in degrees, mainly
because this varies by servo and it's probably necessary to configure
the bounds in some way. Therefore, I added just SetMicroseconds. This
makes the API possible to use and leaves the possibility of adding a
SetPosition in the future.

examples/servo/servo.go

@@ -0,0 +1,43 @@
+package main
+
+import (
+	"machine"
+	"time"
+
+	"tinygo.org/x/drivers/servo"
+)
+
+// Configuration for the Arduino Uno.
+// Please change the PWM and pin if you want to try this example on a different
+// board.
+var (
+	pwm = machine.Timer1
+	pin = machine.D9
+)
+
+func main() {
+	s, err := servo.New(pwm, pin)
+	if err != nil {
+		for {
+			println("could not configure servo")
+			time.Sleep(time.Second)
+		}
+		return
+	}
+
+	println("setting to 0°")
+	s.SetMicroseconds(1000)
+	time.Sleep(3 * time.Second)
+
+	println("setting to 45°")
+	s.SetMicroseconds(1500)
+	time.Sleep(3 * time.Second)
+
+	println("setting to 90°")
+	s.SetMicroseconds(2000)
+	time.Sleep(3 * time.Second)
+
+	for {
+		time.Sleep(time.Second)
+	}
+}