A dedicated performance counter mainly for micro-controllers. For Cortex-M processors, the SysTick will be used by default. The `perf_counter` shares the SysTick with users' original SysTick function(s) without interfering with it. This library will bring new functionalities, such as performance counter,` perfc_delay_us`, `perfc_delay_ms` and `clock()` service defined in `time.h`. A dedicated template is provided to port the perf_counter to different architectures or using a different Timer instead of SysTick in Cortex-M processors.