add some failure tests form arg_pos_at
pass the tests of arg_pos_at
change nsmeaning in bit_at(n) from field of n-1's bits to n's bit.
add a failture test over 32 byte to discitr it failed in 64 bit
pass the bit_at 64 bit test
upgrade pair to tuple to add one more field "pos of arg"
pass the test of add str to msg