Don't inherit from the stl containers

[alekstheod]

https://github.com/KonanM/small_vector/blob/c9bba3e05ded04af2822ac0d2b64d3c322d17be8/include/small_vector/small_vector.h#L68

It would be better to use private inheritance here. There is a risk that someone will destroy the instance through the base class.