seq: enable the fast integer printing code in more cases
* src/seq.c (main): Adjust the initial arbitrary precision
seq_fast enablement checks to be more maintainable, and
a little more general, by allowing single character
separators to use seq_fast.
Also check again after the number arguments are processed,
to see if we can still use seq_fast, which while not
allowing arbitarly large integers, it will handle
integers of the form 10E10 etc.
(seq_fast): Use a specified separator character,
rather than hardcoding '\n'.