Move some Servo functions into IRAM (#1742)

Author: igrr

libraries/Servo/src/Servo.h

@@ -23,7 +23,7 @@
 //   The servos are pulsed in the background using the value most recently
 //   written using the write() method.
 //
-//   This library uses time0 and timer1.
+//   This library uses timer0 and timer1.
 //   Note that timer0 may be repurposed when the first servo is attached.
 //
 //   Timers are seized as needed in groups of 12 servos - 24 servos use two

libraries/Servo/src/esp8266/Servo.cpp

@@ -167,6 +167,7 @@ static void initISR(ServoTimerSequence timerId)
 #endif
 }
 
+static void finISR(ServoTimerSequence timerId) ICACHE_RAM_ATTR;
 static void finISR(ServoTimerSequence timerId)
 {
 #if !defined (SERVO_EXCLUDE_TIMER0)
@@ -180,6 +181,7 @@ static void finISR(ServoTimerSequence timerId)
 }
 
 // returns true if any servo is active on this timer
+static boolean isTimerActive(ServoTimerSequence timerId) ICACHE_RAM_ATTR;
 static boolean isTimerActive(ServoTimerSequence timerId)
 {
     for (uint8_t channel = 0; channel < SERVOS_PER_TIMER; channel++) {