add new API function sv_get_backrefs()

This encapsulates the logic to extract the backrefs from a weak-referent.

Since sv_get_backrefs() can be used for a similar purposes as hv_backreferences_p()
we no longer need to export the later, and therefore this patch also reverts
ad2f46a793b4ade67d45ac0086ae62f6756c2752.

See perl #123473 for related discussion, and https://github.com/Sereal/Sereal/issues/73
for a practical example of why this API is required.