Basic PWM works. Had some problems in testing that might be due to
USART bugs.
HardwareTimer has been removed from the build for now; I will
re-implement it in terms of the new libmaple API, but consider it
deprecated. Let's come up with something better.
Servo is implemented in terms of HardwareTimer, so it also has been
temporarily removed from the build.
pwmWrite() likely got a little bit less inefficient due to
indirection, but the PIN_MAPs shrank by a pointer per PinMapping.
