add SvREFCNT_dec_NN()

add SvREFCNT_dec_NN()

Like SvREFCNT_dec(), but skips the not null check, making code potentially
smaller and faster.

Also as proof of concept, updates the SvREFCNT_dec's in scope.c where
it's obvious the value can't be NULL. There are still 500+ uses in the
perl core that need evaluating!