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