seq: simplify output buffer management

seq: simplify output buffer management

* src/seq.c (seq_fast): Simplify by using an output buffer of
known size (BUFSIZ) on the stack, rather than a heap buffer that
might grow.  For the number buffer, don’t bother appending NUL
since nobody uses the NUL, and xpalloc from nullptr not p0 since
we need to move the buffer data by hand anyway.