add big_sepS_insert_2' and big_sepS_union_2

• big_sepS_insert_2' is like big_sepS_insert_2 but "inserts" into the set from the right rather than the left.
• big_sepS_union_2 is the direction of big_sepS_union that holds without disjointness (for affine assertions). I was quite surprised not to already find this, did I miss something?