std::vec: Add benchmark for .concat_vec and .connect_vec
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]

performance improved according to the bench test:

    before
    test vec::bench::concat ... bench: 74818 ns/iter (+/- 408)
    test vec::bench::connect ... bench: 87066 ns/iter (+/- 376)

    after
    test vec::bench::concat ... bench: 17724 ns/iter (+/- 126)
    test vec::bench::connect ... bench: 18353 ns/iter (+/- 691)

Closes #9581
std::vec: Remove functions concat, connect

std::vec::{concat, connect, concat_slices, connect_slices} are replaced
by the already existing trait methods .concat_vec() and .connect_vec().