A function to calculate the greatest common divisor is generally useful.
In turn declare the internal `frac_long_divide()` as `static`, it's
not used anywhere outside this module and has no public header.
bitarithm.h is not needed for the interface of shed but may cause conflicts
due to different definitions of SETBIT and CLRBIT
common implementations are: (value, offset) xor (value, mask) bitarithm
implements the later
frac.c and nrf52/usbdev.c use bitarithm.h but where missing the include
sam0/rtt.c defined a bit using mask from bitarithm,
changed that to the soulution used in sam0/rtc.c
