<code;name: shiftedmonth.asm
;build: nasm -felf64 shiftedmonth.asm -o shiftedmonth.o
;description: Calculates the shifted month from a given month in rdi, which can be used to calculate
;             Easter sundays (and most probably more than only Easter sundays).
;             The routine doesn't check for legal month numbers, it takes out the lower four bits to calculate
;             with.
;             Returns shifted month number in rax if rdi is a montnumber otherwise rubish.
;  ax