[wpilib] Add functional interface equivalents to MotorController (#6053)

This does not deprecate any current functionality, but prepares the way for future deprecation.

The drive classes now accept void(double) functions, which makes them more flexible.

The C++ API ended up a bit more verbose, but the Java API is really concise with method references, which is >80% of our userbase. For example:

`DifferentialDrive drive = new DifferentialDrive(m_leftMotor::set, m_rightMotor::set);`

Lambdas can be passed to interoperate with vendor motor controller APIs that don't have e.g., set(double), so CTRE doesn't have to maintain their WPI_ classes anymore.

MotorControllerGroup was replaced with PWMMotorController.addFollower() for PWM motor controllers. Users of CAN motor controllers should use their vendor's follower functionality.

wpilibc/src/test/native/cpp/motorcontrol/MockPWMMotorController.cpp

@@ -0,0 +1,31 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#include "motorcontrol/MockPWMMotorController.h"
+
+using namespace frc;
+
+void MockPWMMotorController::Set(double speed) {
+  m_speed = m_isInverted ? -speed : speed;
+}
+
+double MockPWMMotorController::Get() const {
+  return m_speed;
+}
+
+void MockPWMMotorController::SetInverted(bool isInverted) {
+  m_isInverted = isInverted;
+}
+
+bool MockPWMMotorController::GetInverted() const {
+  return m_isInverted;
+}
+
+void MockPWMMotorController::Disable() {
+  m_speed = 0;
+}
+
+void MockPWMMotorController::StopMotor() {
+  Disable();
+}