seq: handle exponents more consistently

seq: handle exponents more consistently

src/seq.c (scan_arg): Set precision and width _after_ exponentiation.
For example, this will make '1.1e1 12' and '11 1.2e1' equivalent.
One can still set the precision by specifying extra precision on
the start value, or more naturally with a precision on a step value.
* tests/misc/seq-precision.sh: Add new cases.