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.
The old headers were moved into folders because doing so avoids polluting
the system include directories.
Folder names were also normalized to lowercase.
