std::vec: Sane implementations for connect_vec and concat_vec

Avoid unnecessary copying of subvectors, and calculate the needed space
beforehand. These implementations are simple but better than the
previous.

Also only implement it once, for all `Vector<T>` using:

    impl<'self, T: Clone, V: Vector<T>> VectorVector<T> for &'self [V]

Closes #9581