seq: support inf last item more generally/efficiently

seq: support inf last item more generally/efficiently

* src/seq.c (main): Call seq_fast for infinite last value.
This implicitly avoids format conversion on the
999999 -> 1000000 transition.
* src/seq.c (seq_fast): Generalize the buffer handling,
and adjust to handle the "inf" last value specifics.
* tests/misc/seq-precision.sh: A new test.
* tests/local.mk: Reference the new test.