seq: use consistent output format with hex integers

seq: use consistent output format with hex integers

* src/seq.c (scan_arg): Set precision to 0 for hex constants
(while avoiding hex floats).  This will use then use the
fast path for these arguments.  Note we also set the precision
of inf to 0 here, which ensures we use consistent precision
on output where possible.
* tests/misc/seq-precision.sh: Add corresponding test cases.