std::scoped_lock was introduced in C++17 and is strictly better than
std::lock_guard as it supports locking any number of mutexes safely.
It's also easier to use than std::lock for locking multiple mutexes at
once.
Timer didn't have working move semantics because mutexes aren't
moveable, meaning the default implementations were ill-formed.
MotorSafety wasn't locking its mutex.
Most of the MotorSafety implementation was moved into the MotorSafety base
class. SafePWM's inheritance of MotorSafety was moved into PWM to
eliminate Java needing a helper class.
In Java, a helper class for Sendable (SendableImpl) was added due to
lack of multiple inheritance.
