Consolidate logic between std_vec and heap_based session implementations.

[davidlenfesty]

Specifically the update function, how it's implemented in HeapSessionManager is much more readable and clear, and we can likely consolidate the logic into a single place.

At the minimum update StdVecSessionManager to mimic the style.

[teamplayer3]

I thought on this as well. I think we can go with HeapSessionManager. This has the same logic with some improvements. It uses the alloc crate, so it can be used in no_std and std environments.

I think a more contrast implementation would be having more static memory usage.