Fix a bug in `seq` where the number of characters needed to print the
number was computed incorrectly in some cases. This commit changes the
computation of the width to be after parsing the number instead of
before, in order to accommodate inputs like `1e3`, which requires four
digits when printing the number, not three.
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.
