GNU seq does not support -t, but always outputs a newline at the end.
Therefore, our default for -t should be \n.
Also removes support for escape sequences (interpreting a literal "\n"
as a newline). This is not what GNU seq is doing, and unexpected.
If we notice that we can represent all arguments as BigInts, take a
different code path. Just like GNU seq this means we can print an
infinite amount of numbers in this case.