[wpimath] Move SlewRateLimiter from wpilib to wpimath (#3399)

Timer was replaced with wpi::Now() to avoid a dependency on other wpilib
classes.

wpiutil/src/main/java/edu/wpi/first/wpiutil/WPIUtilJNI.java

@@ -50,6 +50,10 @@ public final class WPIUtilJNI {
     libraryLoaded = true;
   }
 
+  public static native void enableMockTime();
+
+  public static native void setMockTime(long time);
+
   public static native long now();
 
   public static native void addPortForwarder(int port, String remoteHost, int remotePort);