vec: minimal occupancy rate

[jenspots]

Currently, the vec implementation uses a bitvec to keep track of which elements are present/removed. By doing so, we can remove elements in constant time instead of $O(n)$.

However, we might want to consider having a minimal occupancy rate, such as 50%. When this threshold is no longer met, the elements could be shifted to the front. While this operation would be of linear time complexity, the amortised cost would still be $O(1)$.