Improve the shift_vec function

[YgorSouza]

• Simplified the logic and made it more idiomatic by using the rotate methods that are meant for this.
• Made the panic case more explicit. It is still the same panic as before, but now it can be easily removed if we prefer to just ignore invalid arguments.
• Allowed it to accept any slice, not just Vec.

[lucasmerlin]

Thank you! Looks good