adding #round to Vector as well as Matrix

It looks like @marcandre added `#round` to `Matrix` some years ago, but I would also like to be able to use this method with instances of `Vector`.

Because both `Matrix` and `Vector` both store their components in an underlying `Array`, and implement `#map` over that `Array`, I propose moving `Matrix#round` to `Matrix::RoundHelper` so that it may be included in both the `Matrix` and `Vector` classes and the functionality may be shared between them.