nft_set_pipapo: Prepare for vectorised implementation: helpers

Move most macros and helpers to a header file, so that they can be
conveniently used by related implementations.

No functional changes are intended here.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>