std: rename .connect/.concat in VectorVector to avoid conflicting with StrVector.

This is caused by StrVector having a generic implementation for &[S]
and so #5898 means that method resolution of ~[~[1]].concat() sees that
both StrVector and VectorVector have methods that (superficially) match.

They are now connect_vec and concat_vec, which means that they can actually be
called.