fizzbuzz: Reimplement fizzbuzz-seq in core.logic

Instead of assembling a list of [index fizzbuzz-value] pairs, and
flattening that to a sequence, implement fizzbuzz-seq in core.logic.

This involves a bit of recursion too, but the results are amazing: it
is much faster than the old way, and also a lot more interesting too!

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>