[VecOps] Pre-allocate vectors of the appropriate size in Combinations
[VecOps] Pre-allocate vectors of the appropriate size in Combinations

That's better than calling emplace_back a bunch of times.

olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root