增加舵机库及示例

Digital/servo.py

+from machine import Pin, PWM
+class SERVO:
+    def __init__(self, pin):
+        self.pin = pin
+        self.pwm = PWM(self.pin)
+    def turn(self, val):
+        self.pwm.freq(100)
+        self.pwm.duty_u16(int(val/180*13000+4000))
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example/demo_servo.py

+from machine import Pin
+from servo import SERVO
+from time import sleep
+servo = SERVO(Pin(27))
+a = 180
+while True:
+    a = a == 0 and 180 or a - 1
+    servo.turn(a)
+    sleep(0.1)
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