RTIC Monotonic implementation on a pair of 16 bit timers for various STM microcontrollers.

Select which timer pair to use with features, currently supported: * l0x1-tim21-tim22 - STM32L0x1 TIM21+TIM22 * f0x1-tim15-tim17 - STM32F0x1 TIM15+TIM17 * f0x1-tim15-tim16 - STN32F0x1 TIM15+TIM16 * f0x2-tim15-tim17 - STM32F0x2 TIM15+TIM17 * h743v-tim15-tim17 - STM32H743 rev.V TIM15+TIM17

Other timer combinations are possible as well.